Preserving an entry
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.
Denying an entry (declarer play)
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.
Denying an entry (defender 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.
Rectifying the count
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 tenace and South
wins two tricks. If South plays spades first, she wins only one of the last