What can partner have in diamonds?  He can't have J2, he would have played the J. He can't have QJ2, he would have played the queen. He either has Q2, or a singleton deuce. In either case it is safe to lead the D10 as a suit preference play. If partner ruffs he will return a spade and if partner wins the queen he will return a spade. Foolproof.

Knowing partner cannot throw a queen under a king lead to show a doubleton, allowances have to be made for partner holding Qx.

The winning play on this hand is simple, but somehow not that easy to spot.  If you win the A♣ at trick 1 and lead the Q♦ from hand, then the contract is virtually certain to make.  If the defenders take the K♦ then they can only cash 1 club trick.  If they play trumps, then the J♦ is an entry to dummy to cash the spades and discard any losing clubs.  Of course, if the defenders duck the Q♦ you play A♦ and ruff a diamond in dummy to get access to the top spades.

With an alert partner, it would be 100% correct to play a fourth round of hearts for a trump promotion.  

Why?

Well, if partner held the A♣ he should have ruffed your third heart trick and cashed the setting trick.  On hands like these, you need to trust partner.  If you are still on lead after the third trick, then partner cannot hold A♣ and the trump promotion is the only option. 

Superficially, it appears that you need successful finesses in spades and diamonds. However, there is an extra chance when the full deal is as above:

After drawing trumps, you should cash the K♣, A♣ and Q♣ in that order. When the clubs are 3-3, you can make certain of the contract by leading a spade, covering East's card. This endplays West, forcing him to lead into one of your tenaces.

If you draw trumps and cash the clubs ending in hand, you can no longer make the contract on the above layout - there are not enough entries to extract a 12th trick. If East had one of the kings you could survive only by guessing which king he held, a dubious prospect at best.

When East has four or more clubs you can adopt the same approach in spades, covering the card that East plays. Again, this endplays West and allows you to make the contract when West began with a hand such as ♠K542 ♥985 ♦J875 ♣106.

While you will need the diamond finesse, you can take advantage of East holding either the K♠ or the J♠ and 10♠.

When West has four clubs, you need both the K♠ and K♦ onside. This means you should finesse the Q♠ on the first round of the suit. A point to appreciate is that if you play the 9♠ on the first round, you will fail when West began with ♠J52 ♥985 ♦J87 ♣10865

He will take the 9♠ with the jack and play his 10♣. After ruffing this in dummy you will no longer have the entries to finesse both queens.

After East's play to trick one, declarer paused to consider how to enjoy dummy's diamonds. The only hope was that West had led from a five-card suit, for then there would be a spade entry to the dummy after unblocking the diamonds.

Thus declarer followed with the Q♠ at trick one. When East shifted to the J♣ at trick two, declarer decided it wasn't time for a practice finesse. Declarer went up A♣, and after unblocking the top diamonds from his hand, he led the 3♠ towards dummy.

West rose with the J♠ and paused to consider his options. Eventually, he shifted to the Q♥, which declarer allowed to win - otherwise he would have no entry back to his hand to cash his ninth trick, the K♠. At this point, conceding the inevitable, West took his K♣ and declarer claimed.

Declarer seems poised to run off a bundle of tricks in the red suits not to mention the three spades tricks he has available if he leads low to the 8. Clearly you must shift to clubs and you are looking for four tricks in the suit so you must project some club holding where that is possible. It is barely possible that partner has AQxx in which case you can start with either the king or jack and then play the other honor to unblock the suit and then a little one to partner's AQ. 

Of course things are not always quite that easy. Partner may also have A108x in which case you must start with the jack as declarer has Q9xx. If you start with the king and then the jack, declarer covers with the queen and the 9 morphs into a fourth round stopper. This is a good card combination to remember.  

Leading the second highest card from holdsing such AJx, A10x, KJx, K10x, A9x, K9x, Q10x and Q9x when trying for four tricks  is almost always the right play when the dummy to your right has a small singleton or doubleton. Of course, partner has to realize that you are up to such plays!  

Any discard from dummy on the third heart is worthless, so it may appear that the slam depends upon finding West with the K♣. Declarer had a better idea, setting out to play the deal along elimination lines. He ruffed a spade at trick two, led the J♦ to the queen, and ruffed a second spade with the A♦. A trump to the 7 permitted a third spade ruff, eliminating that suit. Next he cashed the A♥ and ruffed the Q♥, removing that major from the North-South hands.

West had shown up with 2-1 in the red suits, so he was credited with an original 6=2=1=4 distribution, making the contract 100%! Declarer played clubs in the normal way - ace and a club to the 10 and queen. After East won the K♣, there was a pause because East had started with only two clubs and was forced to give a ruff-and-discard. Declarer's club loser was thrown and the ruff taken in dummy, so 12 tricks were made.

If West had turned up with four or more cards in the red suits, Declarer would have led a low club towards dummy's queen without cashing the A♣ first. With West holding the A♠ and probably the QJ♠ as well, he would be unlikely to have the K♣, too. Declarer's idea was to cover West's card, hoping East began with K J x (x), K 10 x (x) or K J 10 (x).

Even if West would never play the K♣ when holding it, this plan offers better than a 50% chance of making the contract. When East wins his club trick from such a holding he is endplayed, forced to concede a ruff-and-discard or lead away from the K (or the 10 or J when West plays the J or 10 on the first round of clubs).

At the table - the deal was played in an international match - South played low, winning the J♣ with the ace. The Q♦ went to the West's king, and another club came back. Declarer played low and East, giving the deal a deeper look, won the K♣  and played another club to dummy's queen. Now declarer was an entry short to establish the diamond suit and then cash winners in the suit.

So, where did declarer go wrong?

As usual - it was at trick 1.

As you can see, if declarer plays the Q♣ at trick one, no matter what happens, there will be another entry to the South hand - the A♣ if West has led away from the king, or the 10♣ by force if East covers the Q♣  with the king. Note that East had to make the correct play on the second lead of clubs. If East ducks the club to maintain communication, South will win in hand and drive out the A♦ with the Q♥ as an entry to the established diamonds.

Cash a third round of hearts, one high club in dummy, followed by the top three spades. At this point, you will have a pretty good idea of East's original distribution. After you cash the two remaining heart winners, everyone will be reduced to four cards.

On this layout, the last heart will force East to make a discard from the J♠, his last diamond and the 1098♣ . A spade discard will be immediately fatal, as will a club, because you will overtake the Q and run the suit.

Therefore, East must throw a diamond, removing his link to the West hand, and allowing you to cash the Q♣  and concede a spade to East's bare jack! East's last two cards will be the 109♣ . You will be holding the AJ♣ . Well played!

When West has one spade and two hearts, you will have to decide whether West began with 1=2=8=2 or 1=2=9=1 distribution. The former is nine times more likely than the latter, so you will need a very good reason not to play for a 3-2 division in clubs.

When partner plays a Queen on your King, that would normally be either a singleton, or would guarantee the Jack so that you can safely underlead the Ace if you need an entry to partners hand.  Clearly neither of those scenarios apply here!  On the bidding, declarer can hardly have a seven card club suit, so partner has a choice of clubs to play at trick 1.  Given that partner has a number of clubs, he could use a range of cards to, for example, show you length using your usual methods, or he might play a discouraging club if he wanted a switch (and you play enc/disc signals), but in either case, the Queen is an unlikely choice.  In this situation, partners Queen is an 'alarm clock' signal - designed to wake you up to something unusual.  The normal switch on this hand would be to a heart as clearly you wouldn't want to assist declarer in setting up his diamond suit.  Without partners Q♣ at trick 1, you would probably cash the other top club and then play a heart, but partner is doing his best to tell you to try something unusual.  That given, you should switch to a diamond at trick 2.  After that, as you can see, partner wins the first trump lead with the Ace, puts you in with a club, and gets a diamond ruff for one off and a huge round of applause.  Unfortunately a flat board as at the other table a transfer sequence put the singleton diamond on lead so now the defense was much easier.

The opening lead tells you that East has one card remaining which is higher than the 5.  You should guess the position correctly by rising with the K♥ as if West held Q975 originally, he should have led the queen to the third trick, as this enables the defense to take 4 heart tricks whenever declarer holds 10x.

However, you might have avoided this guess altogether if you had dropped the 8♥ at trick 1.  This would probably have induced East into returning a heart at trick 2, thinking that his partner had led from a 5 card suit.

The only additional possibility for a 12th trick is to set up a second trick in diamonds. If diamonds are 4-3, generally you will need three entries to ruff diamonds and fourth to enjoy the long diamond. That's the case here:

You begin cashing the A♦ and ruffing a diamond with the Q♠. Now, how can you conjure three trump entries to the dummy? One possibility is to lead the 6 to dummy's 7. When West started with a doubleton 10 in trumps, the 7 would give you your first entry. You could later lead the 9 to the J and finally the 2 to the 4, achieving three entries in trumps.

The main difficulty with this is that when the finesse of the 7 fails you will lose an unnecessary trick in the trump suit, going down even when the K♥  is onside. Secondly, if you lead the 6 intending to finesse, West might insert the 10 on the first round, forcing dummy's jack and limiting you to only one more dummy entry in trumps.

A better idea is to lead the 9 to the jack on the first round of trumps. This benefits you when the 10 is singleton, without giving the defenders any extra chance in exchange. On the diagrammed deal, you win with dummy's J and ruff another diamond high. You then lead the 6 to dummy's 7 and ruff another diamond high, setting up the suit. Finally you overtake your 2 with the 4 and enjoy a heart discard on the long diamond. You can even try for an overtrick by finessing the Q (of course you will be more than happy when the finesse fails).

As the cards lay, your entry-creating play (leading the 9 on the first round of trumps) was necessary to make the slam. So, whenever three trump entries are available to dummy you can establish a diamond for a discard whenever the suit break 4-3.

If there are only two trump entries to the dummy, you can set up a discard (or two) on the diamonds only when a defender began with K Q doubleton or tripleton. Otherwise you must fall back on the heart finesse.

If trumps were 3-0, there would be only one trump entry to dummy and you would have to take the heart finesse immediately after the J♠ won.

You need to pay attention to the spot cards in spades.  East has played the 8♠ at trick 1 (probably from a doubleton).  This means you can establish an extra trick in the spade suit.  Your plan should be as follows:

Win A♠. Draw trumps - it turns out that West has a singleton and East Jxx.  Now play 10♠ from hand.

West wins and switches to the J♦.

Rise with the A♦ and lead 9♠ from dummy, discarding a club.  West can win, put his partner in with K♦ to play a club through - but you rise with A♣, ruff a diamond, and discard your losing club on 7♠ which is now a winner.  This way you make your contract regardless of the position of either minor suit King.  A simple loser on loser play.  How many were tempted into taking both minor suit finesses and then complaining about their bad luck?