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The declarer must establish his long suit, particularly in No-Trumps. Sometimes he must establish a long suit in a hand which does not have any entries outside this suit. If the defence refuses to take the trick which is his immediately, he will often manage to prevent the declarer from profiting from his established suit.

For example:		K J 10 7 3
	9 8 5		A 6 2
		Q 4

South plays the Queen of Clubs: if East takes with the Ace, South can cash in all the Clubs masters thanks to the 4 which he still has.

If East refuses to take (ducking) the Queen of Clubs and only takes on the second round of the suit, an outside entry will be necessary in North to profit from his master suit.

One may have to duck twice:		K J 9 6 3
	7 5		A 8 2
		Q 10 4

This time East must let South take the first two tricks in order to cut his communications.

Notice: once again, this technique is the same one that the declarer uses (ducking the lead, for example), to cut the communications between the two defence players.