When partner leads the Jack its either a singleton or top of doubleton. With South having opened 1NT it looks more like a singleton than doubleton and you must win the first trick and give partner a ruff.

 However when you give partner a ruff, you must tell partner how to get back to your hand so you can give him another ruff.

That’s when we need a SUIT PREFERENCE SIGNAL whereby partner will know where our entry is. You ignore suit being played (here Clubs) and the trump suit itself (here Spades) so that leaves the two red suits. If you now play a high club you want partner to come back the higher of the two remaining suits ie Hearts and if you play a low club you want the lower suit Diamonds.

Since you want a Heart back play the meaningful §9 requesting a Heart back.

Partner is on same wavelength and duly ruffs, followed by a Heart switch. You now win this with your Ace and duly give partner a second ruff. You defeat the contract by one with two Aces and two ruffs.

Had partner not led back a Heart, declarer would have drawn trumps and would make his contract.

You can also view  watch the following movie to see how the hand pans out. (You need bridgvu.exe programme  and it is available free from   http://www.haroldschogger.com/videos.html or BridgeBase programme on your machine already to view it )