South declares four hearts on the spade jack lead. Declarer can be certain the spade ace is offside, so he ruffs trick one and starts drawing trumps. Declarer wants to lead twice toward dummy’s heart honors, so he leads a club to hand in order to make the heart play, running the slight risk of conceding a third-round club ruff.
Say the first heart goes to the queen and king. East cannot profitably continue spades, so he returns a club. Declarer wins in hand and plays another trump to West’s ace. Declarer can ruff the next spade, draw the final trump and knock out the diamond ace. Dummy retains a spare heart to absorb a further spade force.
Observe the difference if West wins the first trump trick: He can continue spades safely, through the void, forcing another of North’s trumps. East wins the next heart trick and lays down the spade ace. Dummy ruffs a third time, but after drawing West’s trump, there are no more hearts left in dummy to protect against a spade continuation. When West comes in with the diamond ace, he can play spades for at least one down — depending on whether West unblocked the spade 10 and spade nine.
It may seem difficult for West to see that he should take the first heart trick, but he should note declarer’s play of crossing to hand to lead hearts. He would simply have played the suit from dummy if he had 10-x-x, and he cannot have a singleton, given his one-no-trump opening.
Here, the reason to win with the ace is to maximize the defense’s opportunities to force declarer.