North must play a low card !   If Declarer holds the ♥J, Dummy's Queen will be captured by Partner's King.
North must hold his ♥A in order to try and capture Declarer's Jack. This way Declarer's side will make no trick in hearts.   Next

If Declarer holds both ♥K and ♥J, North should still play low the first time, and save his Ace to capture either one of Declarer's heart Honours next (or a later) trick.