South’s nonforcing two hearts propelled him to what initially appears to be a no-play game after West kicked off with two top clubs and a spade shift. Barring a miracle, it seemed declarer must still lose a spade and a diamond. However, declarer saw that, on a good day, East might fall victim to a squeeze in those suits.
South ducked the spade shift, so East won with the king and returned a spade to the queen and ace.
Declarer next ran five rounds of trumps. Dummy shed three diamonds as East let go of a club … but then what? If he discarded a spade, declarer could cross in diamonds and ruff a spade to establish the seven in dummy. If East unguarded diamonds instead, declarer would unblock the diamond ace-king before ruffing a spade to hand to score the diamond 10. East had been caught in a ruffing squeeze.
Note the need to play low on the spade shift at trick three. Winning with the ace would cost the contract if the defense attacked diamonds or spades after taking their spade trick. As it went, it would not have helped East to switch to diamonds at trick four, since declarer could reach the same ending but with the spade ace in dummy instead of a second diamond entry.
At double-dummy, there is a defense. West has to shift to spades at trick two. Now the defense has time to remove both a diamond entry and the spade ace. The point is that declarer must give up a club in order to lose the necessary tricks to tighten the position for the squeeze.