Note that to set up the diamonds and pull trump (assuming trump are 3-2 and diamonds are 3-3 or 4-2) he's up to 11 tricks (5 spades, 4 diamonds, and 2 aces). If the heart finesse is onside, then he can get 2 more tricks...but he needs to be careful with his timing. Notice what happened when he didn't win the first spade in dummy and immediately finesse a heart.
If north had held all his hearts (as he probably should have given the auction), then East will be squeezed in one suit (well, one suit and an Ace). I thought this was interesting.

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