They have what appears to be four heart tricks and a trick in each other suit. But the spade trick is the most vulernable, and it vanishes in a puff of smoke. The defense starts with diamonds, west either holding up one round or not. The only vulnerable trick he has is spades, so he'd better set it up. South wins the spade king lead with the ace, cashes the heart ace, and then cashes his diamond, west forced to follow and north pitching his two remaining spades.
South wins and attack trumps, but too late.
West wins the second trump and starts cashing hearts, east pitching spades. If south ruffs the fourth round of hearts, that promotes a trump trick for west. He leads a heart to the king, and south ducks. Whatever east leads, south gets an entry to draw trumps and play ace and another spade. That kills the west hand completely, and the defense still gets two clubs, and the heart ace.