East falls for it
If West had led the Queen of Spades versus Six Hearts, declarer would have had an easy time, winning the King, cashing the Ace-King of Trumps, then running Clubs, and restricting the defence to their Trump trick. However, West’s partner had not doubled the Four Spade control bid and he led the Knave of Diamonds.
Needing to dispose of his Diamond on the fourth Club, declarer