You are West in 3NT and opening leader leads the four of spades (4th highest of her spades). Here is your hand and East’s dummy:
How many tricks do you expect to take in spades? The rule of 11 gives you the answer. By subtracting the rank of the suit led (the four) from eleven, you know how many cards higher than the four are held by you, dummy, and right hand opponent. 11 minus 4 equals seven. You can see six cards higher than the four: A 10 9 K 7 5. That means right hand opponent only has one card higher, so it must be the queen, the jack, the eight, or the six. When you play low from dummy you will know for sure which one it is. If it’s the eight or the six, your nine will win the trick. If it’s the queen or the jack, you will win the trick with the ace, and be in position to take a sure winning finesse against North’s remaining honor.
So, to answer the question, application of the rule of 11 informs you that you have three sure spade tricks.