South bids One Heart, North 1NT, and South Three Hearts. This is a strong invitation to game and as he was up to strength for his first response North should raise to Four Hearts.
West leads Q and South wins with the Ace. South has three top losers and if he plays on trumps at once hemay lose a diamond as well as two hearts and a spade.
He must set up a spade trick quickly for a diamond discard. The Queen of spades is led at trick 2; East wins and returns a diamond, now South goes up with the King and takes the discard on the third spade before touching trumps.