With a choice of winning the first trick in dummy or in your own hand, do not play a card until you have studied both hands and decided which suit you will play at trick two. Once you have made that decision it will often be clear where to win the first trick.
If you intend setting up a long suit, keep entries to the hand with the long suit. Therefore win the first trick in the other hand.
Against 3NT (1 : 1, 2NT : 3NT), West lead the Q. How should South plan the play?
Correct is to win the first trick with the K, not the ace. The suit to establish is diamonds. With five instant winners, you need to set up four more, all of which can come from the diamonds after the ace has been forced out. Therefore keep the A as an entry to your long suit. At trick 2 lead the K. If the opponents do not take it, continue with the J. If the ace has still not been played, continue diamonds. Later cross to dummy with a spade to the ace and play the rest of the diamonds.
South could make three mistakes :
(1) By winning the first trick with the ace of spades. Then if the defenders do not take the ace of diamonds on the first two leads of diamonds (hold-up play by the defenders), declarer will be unable to return to dummy.
(2) By playing low on the first lead of spades (wrong use of hold-up play). South wins the second spade with the king but if the defenders again hold off with the A until the third round (good defenders will use a hold-up play just like declarer does), then declarer cannot cross to dummy as declarer is out of spades.
(3) By leading a low diamond to dummy’s queen first. Suppose the opponents take the A and play another spade. After winning the A, you can play the K and J, but then you cannot return to dummy because you are out of diamonds. You have blocked the diamond suit by failing to follow the high-card-from-shortage rule.