This deal was played in the 2014 Denver Regional. In a Knockout Match, South held (both vulnerable):

K2
A98765
8
AKJ5
He opened 1 and partner responded 4, a Splinter Bid. South has wasted club cards, but still quite a good hand. Should South explore for slam? A good idea is to picture a few possible hands for partner, such as:

A876
KQ42
Q762
3
or Q765
KJ42
AJ76
3
. Opposite those hands, slam is great. Even opposite an unsuitable hand such as: Q765
KQ42
KQ32
2
, you'd still be okay on the 5-level. So I would try for slam, and RKC seems the best tool (no need to make a control-bid). If partner has 2 keycards (either 2 aces or 1 ace and the K), you can bid the slam.

After 4NT, partner answer 5NT. What on earth is that? There are many methods for showing a void after Blackwood, but in most methods, 5NT means 2 keycards and a void. This response should be used only if the Blackwooder will know where the void is. Here, it is quite obvious that it is in clubs (partner's Splinter suit). So, partner has the required 2 keycards (the void isn't of much help), and you bid 6.

A8753 KJ103 9765 --

K2 A98765 8 AKJ5

How should you play after the 10 lead?

If trumps are 2-1, you will be able to draw trump and claim--probably making an overtrick by setting up the spades to discard your singleton diamond.

But, what if trumps are 3-0? If you knew who had the 3 and who had the 0, you would start with the ace or king accordingly. Without knowing, how should you start trumps?

There is no clue from the bidding, but there is a subtle reason to start with the A. Can you see why?

If everyone follows, it won't matter either way. If RHO shows out on the A, you will have a marked finesse for West's Q42. But, even if LHO shows out, you still have a chance. If spades are 3-3, you will be fine (play the top spades, ruff the 3rd round and then throw a diamond on the 4th round as the opponents get their Q). But, the extra chance comes if spades are 4-2. If you lay down the A and find RHO with Qxx and also only a doubleton spade, you are still going to make your contract. How? After getting the bad news in hearts, you play the top spades ending in dummy and then play a 3rd spade. If RHO ruffs (with his natural trump trick), you throw your losing diamond. If he declines to ruff, you ruff and go back to dummy's K and play a 4th spade. Again, if East ruffs you throw your diamond. If he discards again, you ruff, ruff a club and play the 5th spade (now good) to throw your diamond loser.

Why won't this work if you start with the K? Because if LHO has Qxx, you can't benefit from 4-2 spades the wrong way. You have to ruff the 3rd spade and LHO will overruff and cash a diamond winner.

Summary: If spades are 3-3, any play in hearts will work. But, if spades are 4-2 with the person holding 2 spades also holding Qxx, you can still make the hand if RHO has Qxx, but not if LHO has that holding. Accordingly, the correct play is to win the J (throwing a diamond from dummy) and to play the A next. You make on any 2-1 hearts or if West has 3 hearts or if West has 0 hearts and spades behave.

This was the Real Deal:

Vul: Both Dlr: South	A8753 KJ103 9765 --
Q1064 -- KJ43 109876		J9 Q42 AQ102 Q432
	K2 A98765 8 AKJ5

Declarer played correctly (laying down the A). The bad news was only temporary. Declarer turned his attention to spades and Plan B came home.

If you would have laid down the K first, you'd have luckily survived on the actual layout, but not if the East-West hands were reversed.