Right play
Normally Declarer would draw Opponents' losing trumps immediately, playing two rounds of trumps. But not in this case !

Instead, Declarer plays his ♠K. Now Dummy has no more spades left and when Declarer leads his ♠9 on the 3rd trick, it can be ruffed by one of Dummy's two trumps.

Wrong play
If, after winning the first trick, Declarer draws 2 rounds of trumps (hearts) first, there are no trumps left in Dummy's hand to ruff the ♠9 in Declarer's hand.
As a result you will go one down on your contract, losing 4 tricks : one in spades, one in hearts, one in diamonds and one in clubs.