The rule of 7

The rule of 7 says: Add the number of cards in your hand and dummy in the led suit and subtract this number from 7. The result tells you how often to hold up.

I.e. holding Axx opposite xx you calculate: 7-(3+2) = 2 ==> Hold up twice.
The reason is, if you have 5 cards, the opponents have 8. No problem if they are divided 44, but if they are 5/3 you can cut communication by holding up twice.
KQx is better than Axx. You can decide not to hold up, because you have a second stopper, if LHO gets the lead.

If you got 2 stopper in a suit, you hold up once less than with one stopper.
I.e: AKx - xxx doesn't need any hold up (Normally)
7-(3+3)-1 = 0

Links a got from Google typing: "Rule of 7" Bridge
Hold up and ducking strategies Read this to begin with. Definition All the numbers More numbers Great BIL lesson