Keep Your Eye on the Ball

J 8 7 5
10 7
Q J 9 7
10 9 7
2 K 10 4
J 9 6 5 Q 8 4 2
10 3 A K 6 5 2
J 8 7 6 5 4 Q
A Q 9 6 3
A K 3
8 4 Contract: 4 spades
A K 3Opening lead: 10 of diamonds

Let's count out the hand: we see that we have two diamond losers, hearts should be cool, though on a 4-0 trump split, we'll want to make sure we get a heart ruff before leading out all trump. On the other hand, without a bad trump split, we don't wanna take the heart ruff until necessary, since we may need the entry to cash a diamond. We're going to have a club loser if we don't watch out. Can one go on the diamonds? Well, not if the defense attacks clubs from the beginning. But if they don't, we'll want to keep that in mind. And as for spades, there's a 50% chance that the K is offsides in the first place, and if not, there are different ways of playing the spades, but missing the K 10 low, low, we could easily lose a spade trick even with an onsides K. But if we can avoid a club loser, we can afford to lose a spade.
As it happens, the defense cannot hit clubs more than once, since East has a singleton, and West cannot regain the lead. Declarer cannot know that, but when he gets a diamond opening lead, he's one up on the defense anyway. He wants to knock out the K of trump and make use of those diamonds. One difficult moment would arise if East took two rounds of diamonds and led a third. Declarer can't afford to sluff off, since that would be a third trick for the defense. He can only hope that he can ruff high enough to win -- he'd still have a high diamond in dummy -- or to knock out the K. So he ruffs with the 9, and it holds.
He can now read East for both the K and the 10, but he doesn't have enough entries to capitalize on that likelihood. So he lays down the A, then Q of spades, which should have finished off the defense.
Two declarers didn't make out so well, however. The first went low on the 10 of diamonds! She has Q J 9 and she doesn't cover? That's incredible. The Q J 9 with the lead of the 10 -- and a cover -- is the equivalent of starting with Q J 10, which is not too shabby a holding. East ducked, of course, West continued the suit, East won with the K and shifted to the Q of clubs. Ah! He sees your vulnerability there.
Declarer now laid down the ace of hearts and continued low to the J! That was her second mistake. You want entries to dummy. So you wanna knock out the K with the Q so as to draw the last trump with the J in dummy -- giving declarer two entries to dummy. Anyway, East took the J of trump and led a heart, declarer winning and drawing the last trump with the Q. Wherein it should be evident that declarer has but one entry to dummy and needs two: one to knock out the A of diamonds and another to get back and cash the last diamond, sluffing a club. Down one.
The other declarer covered the 10 of diamonds, all righty. East won, took a second round of diamonds and led a third round, declarer ruffing with the Q. Which was a bit too high. Declarer now laid down the A of spades and continued the suit, meaning that he had two spade losers in addition to two diamond. East took his two trump and led the Q of clubs and there the record ends, where it is self-evident that declarer now has one entry to dummy (by way of ruffing a heart) and the top diamond on which the club loser goes.
Could declarer have known not to ruff to high? I'd say so. But you know, even a guy who hates hindsight brilliance doesn't always know if he's been trapped by hindsight or not. A lot of good things could happen on ruffing with the 9. You might win (as declarer would have). You might draw the K, making the rest of the hand a piece of cake. While the worst that could happen is an overruff with the 10. This might mean that the K could be picked up, either now stiff with West, or finessable through E (if that defender has a twice-guarded K), though declarer would be on a guess, of course.
Well, couldn't some nice things happen ruffing with the Q? Um-mm-mm. Declarer wins and now drops the K doubleton on a 2-2 split? Declarer loses, but now picks up the 10 with ease? I don't know. I'll hafta leave it up to the reader's judgment, but it just seems to me that that's too high a card to spend on a ruff, missing the K and 10 among 4 trump out.
Please note also that this declarer, by virtue of covering the 10 of diamonds, would need only one entry to dummy, since East has to spend the A and K to put declarer to that guess on a ruff. This declarer only needs to be in dummy with all trump out to get that diamond winner on which he'll sluff a club.