Here is a hand I observed on BBO. Opening lead is the seven of diamonds. You play low from dummy and East plays the 10. Put yourself in the South seat and plan the play:
Where are your nine tricks going to come from? You count one spade, three hearts, one diamond, and one club. You need three more tricks. A successful club finesse would get you one more trick.
How about the diamonds? It looks to me like you are almost assured of three diamond tricks. Do you agree with me? If so, how are you going to play the diamonds? Give is some thought and then scroll down for the answer.
By “rule of 11” West almost certainly has the ace, jack, and eight of diamonds. Win the queen of diamonds and play a low diamond. That will insure that you get three diamond tricks before your ace of spades is dislodged.