The opening lead cuts declarer down to the same number of trump as West -- and West's going to get the lead with the K of hearts! Another spade lead cuts declarer down to one less trump than his LHO holds. So is that what the West players did? Well, not exactly.
I have the play of two declarers printed out. In one case, the West player shifted to a club -- that's tamely shifted to a club -- upon getting in with the K of hearts! Because what's the use if declarer's gonna ruff? I don't know why, but I've heard that sentiment expressed elsewhere, which of course is the exact wrong sentiment when you've got declarer on the ropes in regard to trump. Declarer was then able to draw trump and when diamonds fell benignly, claim his slam. The other declarer? Did he get forced? Well, not by the defense! He actually forced himself down to one less trump than West before that worthy took the lead! Declarer had led low to the Q of trump, West ducking, ruffed a spade (!) and cashed the heart A, which means he now has one trump left (two ruffs and two leads), while West has the K 9 with nothing higher in dummy and must be able to cash out both. Not a good idea to cut down on your trump holding on such a relatively skimpy holding. I have said many times that an 8-card fit, though usually sufficient to control the hand when you've got good hcp's, is still only a modest holding and cannot make up for a lot of fooling around.
Now lemme look at what I would have considered a more normal line from both the defense and declarer: Declarer ruffs the opening lead, goes to dummy with the J of clubs and takes the heart finesse with the 10, West winning and continuing with spades, cutting declarer down to one less trump than he has. The hand would have looked something like this:
K 3
Q 7
K 5
A Q 2
10
9 8 7 4
9 8 4
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9 8 4
Q 7
9 8
10 7 6
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A J
A J 10 6 3 2
K
Now West has more trump than declarer does in either hand. You might note first off that had declarer used the Q of trump to take that finesse, then West must come to another trick. There's no way declarer can draw all his trump. But if he kept the queen and led to the ten as did the declarer who was allowed to make his contract without a second force, declarer has a counter to that extra trump in the form of a powerful diamond suit. He goes to dummy with the K of diamonds, comes back and notes with relief the Q falling on his right and now he simply runs diamonds. If West ruffs in, declarer overruffs and comes to his A J to run the rest of the tricks. They might get down to something like this:
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Q 7
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A
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9 8
9 8 4
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10
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A J
3
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Declarer must cash his K of clubs early, or sluff all clubs instead of a spade on the diamond differential. In any event, on the lead of the last diamond, West must ruff, and declarer can now overruff and make his contract. Actually, at this point you don't even need to think of being able to overruff. You've got the three top trump and the ability to cash them separately. So the forcing game doesn't work? Wait, not so fast. Declarer doesn't know about the 4-1 split yet, now does he? (I'm referring to my above scenario where declarer goes to dummy at trick 2, finesses the 10 into the K and is forced one more time on a spade lead.) He's drawn only one round of trump. A 3-2 split is about twice as likely as a 4-1. On the former, the best thing he can do is to cash out his trump and run his winners. Do you really think any declarer, after one round of trump, would pursue diamonds? Just because we can see that it works? I doubt that very much.
So granted, this isn't an open-and-shut case. The first declarer did finesse the 10 into the K and had he been forced with a second spade lead, just might have found the winning line. But it's very unlikely, and surely the continued pursuit of spades, forcing declarer to ruff, would be the most threatening line of attack, the likeliest to allow the defense to rake in a second trick.
Footnote: Well, gosh, couldn't declarer have withstood the forcing game in diamonds? With that six-card suit? Indeed, that would seem to be the case. He ruffs, draws trump in three rounds, leaving him with two more trump when he really needs only one, and knocks out the K of hearts one way or another. So, it's contrary to two of the most common guiding principles of bidding: choosing the major over the minor and choosing the more balanced over the unbalanced. The latter doesn't work to any advantage here because declarer has all top winners aside from the K of hearts. And as for the latter, well, that's the luck of the draw. Give East the K of hearts with one kicker or two, and you've got a much better score in the major than in the minor. And I dare say that the declarer allowed to make 6 hearts (matchpoints 87%) got a much better score than those in diamonds, though I didn't think to check. In any event, that's not the way we play bridge, to bid on the basis of what you know later. Choosing the major over the minor (assuming at least 8 cards in each) will benefit you 10 or twenty times for every time a post mortem shows that the minor would have worked better. However, I do not recommend an inadequate major over an adequate minor.
In any event, I wouldn't be ashamed or disgusted with myself to wind up in 6 hearts here and get tapped down to a losing contract. Hey, no one's going to get 'em all right. You've gotta go with probability for the long haul and accept the disappointments with a shrug.