South took the trick with the club ace and ruffed a diamond to eliminate the suit. He then laid down the spade ace. He knew that if the spade king was onside, he did not need the finesse (barring a void with West); East could be endplayed with the spade king later on, after the hearts had been cashed. A 3-0 spade split seemed unlikely, given West’s apparent club shortness. Meanwhile, this line kept declarer in the game if West had the spade king.

Declarer then started on hearts. If East had started with a singleton heart, declarer would have played a trump right away as planned, but after East showed out on the third round, South paused to reassess. Diamonds were presumably 6-6, given the bidding, and West had shown up with three hearts, leaving him with four black-suit cards. Assuming West had short clubs for his lead, he had to hold the spade king. Declarer therefore cashed the club king before exiting with a trump, endplaying West to concede a ruff-and-discard. Away went the club loser from the South hand, and the slam was made.

Would declarer have pursued this line on a red-suit lead? I think not. He might still reject the trump finesse, but he would then exit with a trump after eliminating the reds, leaving the club king in dummy; West’s distribution could have been 1=3=6=3, for all declarer knew.