After leading your ♦7, your 4th highest of your longest suit, partner wins first trick beating dummy's Queen with the king and firing the Jack back. Make sure you overtake that Jack so you can cash all your winning Diamonds now. While you are cashing the diamonds, partner plays a low club followed by a low heart telling you not to play either of these suits.

So after cashing all the diamond, you dutifully play a spade and with partner sitting over dummy with the AQ10 he now takes two more spade winners and you defeat the contract by 3 tricks.

(If initially you had led a spade and partner were clever enough to switch to King and another Diamond and you overtook the third round, you would cash all your Diamonds and then lead a second Spade and then the contract would go 4 down BUT then you night be accused of cheating, rather than hindsight)

NB with a red suit lead declarer has no trouble making 10 tricks as long as he plays Hearts first and then enters dummy with that ♠10

you can watch a film of the hand being played out by going to