To force an opponent to play a master card (eg ‘To knock out the ace’). In a no-trump contract, it is common for each side to knock out its opponents’ high cards early in the play.
South plays in 3NT. West leads the ♠6, which ‘knocks out’ the ♠K. Declarer plays on hearts, leading high cards from dummy to ‘knock out’ West’s ace. The defenders then play a second and (if South ducks) a third round of spades to knock out South’s ♠A.