The defense has four obvious tricks, and the only real possibility for two additional defensive tricks is in hearts.
Diamonds are possible, but west has no entry.
East can set up one long spade, but that's only one trick. With west on opening lead, he obviously cannot attack hearts. So instead he heads a diamond to east's king, and east shifts to a heart. The king of spades is led off dummy, and east ducks two rounds then wins the third, and exits a heart. South plays low, west plays low, and the ace in dummy wins.