As East today, you bid one spade over North’s diamond opening. South cue-bids two spades to show a good raise. North’s three spades sets up a game force and asks South to bid three no-trump with a spade stopper. (Three clubs would be a better call and might lead to a contract of five diamonds — though the hand is not easy to judge.)
Your partner leads the spade two against three no-trump, showing count rather than promising an honor. You can infer that he is likely to hold three small spades, not four, based on his failure to act in the auction. Your spade 10 holds the first trick. What next?
Looking at the dummy, you can see six diamond tricks and a spade trick for declarer. Partner must have the club ace, or South would have cashed out already, which means South must hold the heart ace. If you pursue the routine defense of leading the spade king next, declarer can win the third spade trick and is likely eventually to play on clubs — whereupon partner will have no way of reaching the spade winners in your hand.
You must look to the heart suit instead, but only partner can attack the suit profitably. It follows that you need to get partner on lead with a second round of spades so he can play a heart through.
Steel yourself to underlead your spade K-Q-J at the second trick! Your hope is that partner has the spade nine and heart jack. Declarer must duck to cut the link in spades, after which your partner should deduce that he is being asked to switch. A heart shift from partner defeats the contract.