Declarer is holding up his ♥A until the 3rd trick, trying to isolate West's long ♥ suit.
If East wins the 2nd trick with his ♥K he is forced to lead another suit. Consequently Declarer can hold on to his ♥A stopper, which will greatly enhance his chaces of making the 3NT contract.
If East holds ♥ K 7 3, he should play the ♥7 on the 1st trick, but play the ♥K on the 2nd trick (in order not to block the suit at trick 3), so that he still has his ♥3 to lead at trick 3.
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