There are no side entries to the North hand. If declarer plays ace, king and another, East will win the third trick. The remaining two small cards are good, but there is no way to get to them.
Proper procedure is to duck the first trick. Then, when the lead is regained, playing the ace and king will establish the suit and the remainder of the suit can be cashed.
Note that defenders can benefit by ducking as well. A defender with a holding like the above (AK432) and no outside entries may do well to duck the first or second round of the suit.
West leads the top of a doubleton against a suit contract, and East plays the queen. Suppose South wins the first trick with the ace and West gets in before trumps are drawn (with the ace of trumps, for example). Now West can lead her remaining card in the suit to East's king, and East can return the suit for West to ruff.
Proper procedure is to duck the first trick, and win the ace on the second round of the suit. Now, when West gets in, she is void in her short suit and cannot use that suit to get to East's hand.
Ducking to deny an entry when playing no trump is known as a holdup play.
The declarer (South) plays toward dummy's long suit. Assuming there are no side entries, on the distribution shown East must duck once to prevent declarer from running the suit.
Note that West must give a proper count signal in this situation. In the distribution shown, West signals an even count; East assumes it shows four and ducks once. (If West has only two, then South has four and ducking neither helps nor hurts.) If West signals an odd count, East will have to decide (possibly from the bidding or previous play in other suits) whether it shows three or five, and win the first or third trick accordingly.
Often, to set up a squeeze, one or more tricks must be lost in advance. This is known as "rectifying the count", but it is actually just another instance of ducking.
Here is a simple endplay situation:
At no trump, South is on lead with three cards left to play. South ducks a heart to West, who must now lead spades into South's ace-queen fork and South wins two tricks. If South plays spades first, she wins only one of the last three tricks.
A defender can deceive the declarer by ducking a trick he could win, inducing him to place the defender's cards wrongly and adopt a losing line of play. In the position as on diagram, the declarer leads a small card from dummy to the queen in trump suit. If West takes the ace, the declarer could easily play the king in second round, dropping East's jack. However, if West ducks, the declarer will place the ace with East, and finesse the dummy's ten in second round, losing to East's now singleton jack.
This kind of duck requires perfect reading of cards and nerve, especially if the suit involved is a side suit in trump contract. East must duck smoothly, without hesitation, otherwise the declarer could easily smell the fraud.
Pour réussir son contrat Nord doit affranchir sa longue à♣ et se prémunir contre un mauvais partage de cette couleur (Valet de ♣ 4ème). Nord prend l'entame du Roi de ♥, débloque As et Roi de ♠ et avance le 4 de ♣ avec l'intention de fournir le 3 de ♣ du mort, manuvre du "COUP à BLANC".
Si les ♣ sont 4/1 chez l'adversaire le coup est réussi. Quelque soit le retour, Nord en main, joue le 5 de ♣ pour la Dame du mort, avance la Dame de ♠, reprend la main à ♥ ou à ♦ et défile tous ses ♣.