A play made in an attempt to cause a blockage in the opponents’ suit. For example:
West leads the ♠6 against South’s 3NT. The contract is in danger if spades split 5-2, because the defenders might make four spade tricks and the ♦A. Since you would expect West to lead the king from K-Q-J-x-x, you can place East with at least one of the missing honours. In this case, if spades are 5-2, the play of the ace from dummy will block the suit. East cannot play the queen without giving declarer a second stopper. Similarly, when one of them gains the lead, they cannot afford to play the king and queen on the same trick without promoting South’s holding into a fourth-round winner.
