Forcing NT convention is accepted from 2/1 GF system, but in sayc has the same values. This can manage much problems after 1Major openings. The gadget tell you, that after partner's 1Major opening the 1NT response is forcing (for one round only!). So after 1Major opening you cannot play 1NT (only, if 1H-1S-1NT bidded by us, without interference). But this is not too expensive: the values of forcing NT are much-much higher.
You can bid 1NT response as forcing either with trump support or without. No point limit. Why is it useful?
A/ With trump support: imagine always 1S opening. With QJx Kxx Ax Jxxxx bid 1NT first, then jump on 3S. What is the difference between 1S - 3S and 1S - 1NT - 2any - 3S? You see: the immediate limitraise promises 4+ trumps, the limitraise after 1NT forcing shows same in hcps (11-12 or a good 10), but exactly 3-card trump support. With QJx Kxx AK Jxxxx bid 1NT first, then jump to 4S: and with forcing NT you can avoid 2C first response with a not too attractive 3-losers suit.
With semi-support: xx Axx QJxxx xxx 1S - 1NT - and if p bids on 2nd level (not 2NT), then you can correct on 2S, showing the 2-card support (or do it as correction with xxx KQJ xxx xxxx as well; the immediate 2S response is stronger as 1S-1NT-2any-2S).
B/ Without support: simply bid 1NT with anything when less 10 hcps. With 11+ produce a 2 over 1 bid (the good 10 is 11, the bad 10 is not!), if you have an 5-card suit or a good 4-carder (better to do a 2 over 1 with a very good 4-carder, as AKxx or KQJx, etc.). With 11+ and without a good suit, bid 1NT at first, then 2 or 3 NT second time (or raise partner's second suit with 4-card support as second bid).
After forcing 1NT the opener must show full strength (jump shift or 2NT with invitational or better hands), because over forcing 1NT, if it was weak and without support, opener's weak rebid is PASSABLE, even if it is a new suit! So with x Kxxx Jxxx QJxx: 1S - 1NT - 2any - PASS; but with the same hand 1S - 1NT - 3H - 4H, bingo... Sometimes very hard to bid over 1S, if you have less 10 hcps and good clubs: 2C promises 11+ and still forcing, 1NT can go down in weak red suits (and passable, if this convention not used) - with forcing NT you cannot stop in 1NT, but can stop in 2C is partner second time bid this. Interesting, that 1Major opener must not rebid 5-card major (only with AKQJx allowed); have to reply with better minor if has not 4+ hearts. This minor often 3-card long only, so if weak responder want to pass, not the best choice with 3-card support of this minor: easy to "find" the 3-3 fit. With 2-card support of opened major and weak hand nearly always the best rebid of responder is the correction to 2Major.
C/ In opposite: when you do not use 1NT response, I can imagine better your responder hand. 1Major-2Major means 3+ support 5-9 hcps. Limitraise (1M-3M) always 4+ trump support, and 11-12 points (dummy points counted: this means in my system hcps+(trump length-shortest suit length). 1M - 1NT - 2 any - 2NT means: I have not 3+ in your major, have not a 5-card other suit or a good 4-carder; but have 11-12 hcps balanced (for example 2344 shape, most likely with stoppers in unbid suits. 1M - 1NT - 2 any - 3NT: the same, with 13+ hcps. 1M-1NT-2x-3x: I have 11-12 and 4-card support in x suit. So with forcing NT you can better know the trump support length, and can describe the hand better.
My system (created and published by ng) over 2C strong openings. (For 2C needs 22+ hcps, or minimally 9.5 own tricks - mostly with high cards).
2D: negative or waiting (70-80%). Not always bad hand, and does not deny aces! Positive answers quite well defined, so if the responder's hand not exactly the defined positive one, must bid 2D at first.
2H and 2S / 3C and 3D: positive. Minimally 5-card suit with A or K, and 3+ controls in hand (ace=2 control, K=1 control). In minors mostly 6-card suit or longer needs (with better 5 -card minor allowed - as AQ10xx).
2NT: 8-9 hcps balanced, but this bid denies any ace!
3H and 3S / 4C and 4D: positive, 6-card or longer solid suit (AKQJxx or AKQxxxx minimally).
3NT: 10-11 balanced, this bid denies any aces either!
4H and 4S: weak 7-8 card suit without side suit strength (for example KJxxxxx), no 3 controls.
The opener's first rebids after 2D response:
2NT: 22-23 balanced all suits stopped. This is passable by responder with 0-1 (or a bad 2) hcps and bad hand. This is the first way to stop below game.
2H, 2S, 3C, 3D (suit rebids without jump): 5+ suit and forcing. The second negative answer from responder is 3C, or (after 3C or 3D) the next lowest possible suit bid (3D and 3H). Then, if the opener simply repeats his suit without jump, responder with nothing can pass - all other rebids by opener are forcing.
3H, 3S, 4C, 4D: (suit rebids with jump): all these are GF and show solid suit which will be the trump (even if responder has void). Then natural bids and keybids are coming - responder's immediate raises on game are the weakest second answers, all else show some values.
3NT, 4NT: 24-26 and 27-28 balanced all suits stopped.
The opener's rebids after positive responses, and responder's tactic:
The immediate raises on game always the weakest bids - others are natural. (This is the same with responder: when raises opener's suit immediately to game, shows nothing and wants to sign off.) Jump shift by opener means the same as his jump shift rebids after 2D: own suit which will be the trump even if responder is void. Remember, that there are 3+ controls when responder gave positive answer without jump (except 3D - this is the same with jump). For example, with AK xx AKQ AKQJx after 2H positive response the opener can bid 7NT immediately because the responder must have H AK.
Kantar (checkback Stayman):
after 1 minor - 1 Major - 1NT the 2C rebid from responder is an asking, with minimally invitational strength (may be stronger as well) - so needs a 11+ hand to use it. There are different answers available from opener.
If responder's first bid was 1H, then simple:
2D: denies 3-card heart support and shows weak hand (13-14 high card points
2H: promises 3-card heart support with minimal opener;
2S (facultative, not all players use): no 3 hearts but medium opener with good spade stopper, invit for 3NT
2NT: denies 3-card heart support but promises medium opener (14-16 hcps)
3H: 3-card heart support and 14-16.
If the responder's first suit was 1S, the heart suit is interesting too (responder might have 5-4 in majors, and opener might have 4-card H suit too, but should not rebid it because it would have been reverse, and not enough points for a reverse). So answers over 2C now:
2D: no 3 spades, no 4 hearts, minimal opener.
2H: no 3 spades, 4 hearts yes, minimum.
2S: 3 spades with minimum.
2NT: no 3 spades, no 4 hearts, medium hand (14-16)
3H: no 3 spades, 4 hearts yes, medium hand
3S: medium hand with 3 spades.
Please notice, that using Kantar always the 2C is the asking bid (never "unbid" or "other" minor, so never diamonds - after 1C-1H-1NT the 2C is asking, not a correction on clubs!). Why? I think because the 2D arteficial response needs for safe stopping: if the 2D would the asking bid after 1C-1H-1NT-2D, what is the negative answer below 2H? The only problem, that the pair cannot stop in 2C with weak hands (for example 1C - 1H - 1NT : now the responder with x xxxx KQx Jxxxx must not bid 2C as correction and stopping - can stop only in 3C, or can pass the 1NT which is not the best here with both short spades). The partnership can agree if after 1C - 1M - 1NT the 3C is weak (named - as I know - Walsh-stopping), stopping with unbalanced hand 5+ clubs - the 1C - 3C inverted raise was not available immediately because of the 4-card major.
(Other thing is that this method is "competing with hoki-style": does not need immediately raising responder's major with 3-card support, you can find the 3-5 fit as well, when the responder has more as 5-10. When the responder is weak, the hoki-raise can be dangerous, if a 4-3 fit found...)
The New Minor Forcing (NMF) in Hungary:
after 1minor - 1 major - 2 same minor (repeating the opening suit) - 2(3, if clubs) other minor is the NMF.
So here is not 1NT rebid: opener repeats his suit on 2nd level, and shows minimal or medium opener with 5+ minor suit (at least 6, or a good 5). The forcing always the other minor. There is a difference between 1C-1M-2C-2D and 1D-1M-2D-3C: the second is stronger, always GF (first version might be only inviting). There are 4 sequences: 1C - 1H - 2C - 2D, 1C - 1S - 2C - 2D (these are invitationals, responder promises 11+, and the 3C answer passable - 2NT too, it shows minimal opener with unbid major stopper, but no 3-card support in partner's major). 1D - 1H - 2D - 3C and 1D - 1S - 2D - 3C are both gameforces.
The answers are similar as in Kantar: repeating the opening minor thirdly is the weakest answer with long minor, no major support, no 4 hearts (if the responder bidded S at first) and no stopper in unbid major. 2NT and 3NT answers indicated with stopper in unbid major.
I think this system is correct: and this checkback Stayman cannot be called NMF, because often not the new, but the opening (clubs) minor means "forcing". It is near to Stayman, you want to find a major fit after an 1NT bid - either 5-3 in bidded suit or 4-4 in unbid hearts (no chance for 4-4 spades after 1m - 1H - 1NT, because the opener should rebid 1S with 4 spades).
Ogust convention mostly used after 2M (major) weak opening.
See, that it works well after old-fashioned, conservative 2M, in which always
there is minimally 1 high honor (A, K or Q), and the opened suit is correctly
6-card long - and opener's hand has 6-11 hcps at least. Nowadays some experts
use the weak openings with much worse hands and even with 5-card suit if p has
passed already. In BIL level the conservative weak 2M is the best - with weaker
hand the pass is the best bid
I use weak 2 openings with quite weak hands, when partner passed yet - but then his/her best choice not Ogust (if has something even if passed) but raising the weak 2 (possibly with jump). After a weak 2 in all cases RONF played: Raise Only NonForcing - and, over raise opener's pass is indicated, so raise as high immediately, as possible.
The bidding is something like this: 2S (weak) - pass(opp) - 2NT (Ogust-question, asking for feature). With agreement Ogust is STILL ON over opponent's dbl (always off over opps suit overcall). There are step answers:
1./ 3 Clubs: 1 high trump honor and minimal hand. About: S: K109xxx, H: QJx, D: J10, C: xx.
2./ 3 Diamonds: 2 high trump honor and minimal hand. At least S: AQxxxx, H: Jxx, D: xxx, C: x.
3./ 3 Hearts: 1 high trump honor and maximal hand. For example: S: A109xxx, H: Qxx, D: Kx, C: Jx.
4./ 3 Spades: 2 high trump honor with maximum (sometimes a borderline hand between 1S and 2S opening). Something like this: S: AQxxxx, H: x, D: KJ10x, C: xx.
5./ 3 Notrump: AKQxxx in trump suit (and, of course, possibly nothing or maximally a Q in other suits).
You can ask Ogust-question after 2S opening with: S: Jxx, H: Axxx, D: KQx, C: QJx. See, that the best way of using this method a notrump-like hand - all other suits are stopped. If you do not avoid using with xxx in one side suit, the weak opener might have the same xxx and if you bid a game, will fail because opponents can call immediately 3 fast tricks in your weak suit.
The other reason: opener must be disciplined! Responder asks, so only the responder knows the best contract - opener must pass everything except 4NT Blackwood!
If weak 2D was opened, the 2NT question may be used too. But the answers are different, because the first target is 3NT with minors. Responder must have notrump-type hand (all suit stopped, opening strength or more) for asking: 2D opener promised only 6-11 hcps. The answers are:
1./ 3 Diamonds: negative response, bad hand, bad suit, discouraging, asks for pass.
2./ 3 any other suit: positive response, better hand and stopper in bidded suit - accept trying of 3 NT (the responder must decide about it - possible other signoffs are 4 or 5 -sometimes 6- diamonds).
3./ 3NT response: same as after major Ogust: AKQxxx in diamonds (and most often nearly nothing else).
The whole Gambling 3NT convention, as I know (or, as ng - Gábor Nagyiván - wrote it in his book "Bridgeschool"). May be others are playing with other options; for me it is good and logical.
So: needs a solid minor suit: 7 card or longer. Like: AKQJxxx or AKQxxxxx (AKQxxxx is dangerous: may be an opponent has Jxxx, but can try with). Other conditions: no 4-card major suit, and in side suits maximally one Q (no king and ace). No void! No two singletons (sometimes we use it with two singleton or a void...)! A typical gambling hand: Spades: Q109, Hearts: Jx, Diamonds: AKQJxxx, Clubs: x. Or: xxx xx x AKQJxxx is good for it too.
With this hand you can open with (gambling) 3NT. In agreement g3NT can be used after opps opening (as a jump), but needs partnership agreement for this.
Answers of the responder:
1./ Pass: to play! Partner has stoppers in other 3 suits, but NOT Kx or Qxx - gambling opener will be the declarer, so Kx (Kxx)-like suits are dangerous, if ace is behind partner, and that suit led. Good hand for pass: Spades: Axxx, Hearts: KQx, Diamonds: QJx, Clubs: xxx. Sure, that gambler's suit is Clubs, and there are 9-10 sure tricks. (Responder must not be void in gambler's suit)!
2./ 4 Clubs: escape in gambler's long suit. If this is clubs, gambler must pass, if diamonds, correct on 4D.
3./ 5 Clubs: same as 2./, but stronger (unbalanced) hand, no stopper in one or two suits, and wants to play a minor game. Pass or correct bid: if the long suit is Clubs, gambler bids Pass, if Diamonds, 5D. Good hand for 5 Clubs response: Spades: AQJ10, Hearts: KQJx, Diamonds: xx, Clubs: xxx. May be down 1 (with bad luck 2), but better to try minor game with, but 3NT out of order because of the unstopped other minor.
4./ 4 Diamonds: responder KNOWS which is the gambler's minor suit (because in other he has a figure, as A, K or Q). This bid asks gambler's singleton! Why? Want to see the fit of hands. For example responder has this: Spades: Axxxx, Hearts: AKJ, Diamonds: KQ, Clubs: xxx. With this hand, if gambler has singleton spade, 6C= sure (very good fit), but if the gambling opener has xxx in spades, 5C will be the good contract. Gambling opener's rebids on 4D question: 4H shows single heart, 4S spade, 4NT single (other) minor, and 5m (the long gambling suit): no singleton, stopping in minor game. If opener has shown a singleton, responder must to bid the final contract (he asked, so he knows more).
5./ 4 NT: responder asks the length of gambling suit (and KNOWS which is that suit, as in case 4./). This is a slam-interesting ask. Answers are: 5Clubs with 7-card suit and 5 Diamonds with 8-card (or longer). Why asked the responder the length of gambling suit? See this hand: Spades: Axxx, Hearts: AKx, Diamonds: Axxx, Clubs: xx. If the gambler's suit contains 7 cards (rebids 5C), I can discard with this 5 small cards on 5 extra clubs (with which the gambler's hand is longer as this), but I have 7 low cards in side suits, so I shall PASS the 5C rebid. If the rebid is 5D (8-carded, or sometimes longer), I can discard 6, and I have all side suits ace, so I can bid 6NT, which is surely made. If the gambler's suit is longer as 8, he can raise 6NT on 7, but it is dangerous sometime (may be one of my stoppers is only KQ). Of course 4NT is never Blackwood: no reason in asking aces, when responder surely knows that gambler has strictly one ace (and nothing else in other suits, maximally a Q).
6./ 4 Hearts or 4 Spades: to play, asking for pass! Shows long and very good major suit (can play with singleton support!). Do it with (for example) as 4 S: Spades: AKJ109xx, Hearts: Ax, Diamonds: KQx, Clubs: x. Maximally 1-1-1 S, H and D losers (P must have solid Club suit, is you can discard on that, you can get some overtrick as well).
And this is not too hard, as I know, but quite good, logical and nice for preempting opponents. Very hard to find their major fit after 3 NT opening (and very hard to find the possible slam). If somebody uses this, must not open 3NT with 24-27 balanced: must open 2 Clubs and rebid 3 NT with.