[smerriman]

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On the 4th trick, West must play precisely the 7 from T73 to defeat the contract, despite the fact that the next round of the suit will be won by South's remaining stiff Ace. What exactly is going on here?

[sfi]

I haven't looked at it that closely, but you obviously can't play the 10 because that gives declarer 9 tricks. But you want to avoid being endplayed later in the hand, and declarer has the option of playing the spade ace and unblocking the 9 from dummy to throw you in. Ditching the 7 deprives declarer of that option.

[smerriman]

Ah yep, you're right. You can't afford to throw the T before declarer leads to the ♦J, but can't afford to win a spade trick after declarer leads to the ♦J. So have to keep both options open to avoid the clever spade play to endplay you in hearts later.

Bridge is fascinating sometimes.