1 - Fourth Suit Forcing to game
2 - RKC Blackwood
3 - 2 Keycards without the trump queen

You are East on this deal, on lead against 6♥. First, a quick comment about the bidding. The 1♠ bid by North is a terrible choice. The 4-3-3-3 hand pattern along with no intermediate spots warrants a limit raise. Having chosen to force to game, a two-over-one game forcing 2♣ is a better call than 1♠, as it will simplify the rest of the auction. In any case, the auction has revealed a lot about declarer's hand.

Against high-level contracts, your partnership has the useful agreement of leading the king from ace-king to ask for count, so you lead the ♠K. Partner plays the ♠10, showing even number of cards in the suit. What next?

Solution:
The bidding tells you that declarer is 1-5-3-4, and partner's signal confirms that declarer does have a singleton spade. Declarer is unlikely to have a club loser (he can finesse partner's potential honor. Only when his holding is the AJ10 does he have a guess in clubs), and he is extremely unlikely to be missing the trump queen, since he proceeded to slam after his partner denied the trump queen. If he does not have a diamond loser either, then the slam is cold, so you must assume that partner has the queen or king of diamonds. Since you have the 1097 of diamonds, attacking with the ♦10 is a logical choice, and cannot cost a trick. It actually looks as if declarer cannot make the slam regardless of what you return, since he has an unavoidable diamond loser. That is not completely true - declarer cannot make the slam as long as you don't play a second spade.

If you play a second spade, declarer can succeed with the help of a dummy reversal. Declarer will ruff the spade continuation in hand, play ♥A and a heart to the king and ruff a third spade in hand. He will then play a club to the King, ruff the last spade with the ♥Q to complete the dummy reversal. He can then then cross back to dummy by playing a diamond to the Ace, cashing the J of hearts to draw the last outstanding trump, and claim the remaining 4 tricks with the AQJ of clubs and the K of diamonds.

Analysis:
Bridge Baron's double dummy solver agrees that any defense except playing two round of spades will set the contract.

Bridge Baron's Line of Play
Bridge Baron started out with the normal ♠A lead. Baron took its time to think about the play to trick two, and switched to a trump to defeat the contract. This is a technically sound play, since this will foil the timing required for any dummy reversal in similar situations (although on this deal a club or a diamond work too).
Par Contract Analysis:
The par contract on this deal is 5♥ by North-South.

Bridge Baron deal No : N3712-42870-24243-43348-40897-50169