West led ♠7, declarer played low from dummy, and East….. East had already made the reflex “third hand high” play of ♠J. Declarer won ♠K, drew East’s trumps finishing in hand, and played ♠3 to West’s ♠9 and dummy’s ♠10. When it held the trick, he was able to discard ♣Q on ♠A. He just lost two ♥s - trumping his third and fourth ♥s in dummy.
If East had not played ♠J at trick one, the contract would have gone one down. Could he have worked out to play low? The answer is an emphatic yes! West could not have both ♠K and ♠Q - or he would have led ♠K. There is no other holding where it could gain East to play ♠J, and plenty where it would lose heavily. Like the actual hand!

ANDREW’S TIP: Think before making the reflex play. When defending, work out partner’s possible holdings in the suit he has led.