Declarer should win the Ace and draw trumps, ending in dummy. Usually one hopes to win finesses but here declarer leads a diamond to the Queen and prays it loses. If it does West will be endplayed, forced to concede a 12th trick in a minor. If the Queen of diamonds wins, South has to continue with Ace and another, hoping either that East started with Kx or that West found a clever duck holding the King.
Of course, to have any chance declarer needs the King of clubs in the West hand. When the hand was played South thought he could test that early. He cashed the Ace and King of trumps, played a club to the Queen then took the diamond finesse.
South was expecting West to win and exit in trumps, allowing him to execute a double squeeze. He tests the diamonds, finding that West has a guard, then runs the rest of his trumps. West must keep a diamond and East a top spade, so in the ending neither can hold two clubs and dummy's four of clubs takes the last trick. But West threw a spanner in the works.
He got off lead with the King of clubs, taking out the club Ace entry and breaking up the squeeze.