Capture that Card!

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

A strange hand. Some East's opened the bidding with one spade, which didn't hinder their LHO's from winding up in four spades. And those who did, i.e., those Easts who did open a spade directed some declarers (when East went low on a trump from dummy) to go low themselves, winning the first round of trump with the 6! Other Easts, without a spade bid or a double, went up with the A, first round. Now why on earth would one wanna go up with the A, for heaven's sake. As if that winner is going to escape them? Surely you'd wanna duck that lead and let declarer come to you with your not-too-shabby cards above the deuce.
The upshot was, that the hand became pretty much a case of whether East went up with the Ace, on that singleton lead from dummy or did not, the former allowing declarer to make his contract, the latter moving toward a defeat -- by and large.
But here's a defender who didn't commit that foolishness, nor did he bid a spade, nor double the contract, and so ducking that lead, declarer naturally enough went in with the 9 -- and saw his LHO show out!
That was trick five, after two club leads and two heart. Tricks 6 & 7 were two diamond leads with the closed hand's honors, followed by the K of spades to the Ace, and a third club lead from the defense, declarer ruffing low, and now the 10 of spades, East ducking! ! ! Now there was a diamond to dummy's Q, and a heart lead, trapping East's 8 of trump, whether East went up or not.
East could do nothing at that point, but why on earth duck the 10 of spades. The hand looked like this at that point:

------
J 3
Q
9
------ Q 8 7
Q 10 8 ------
------ 10
Q ------
10 9 6
------
8
------

Wherein it should be evident that if East would simply take that 10 with the Q, he would then have another winner coming with the 8 7 of trump battling declarer's 9 6. Should East have known that? Well, I'd say yes, unless you'd want to excuse East from watching the only suit he much say in. But I'd go further. Even if he hadn't watched spades very carefully, where on earth is the percentage in ducking? East had properly ducked the low trump lead from dummy so that declarer would come to him, as he did with the K of trump, properly won by East and is now doing so with the 10. Do you want a better card that the 10? Oh, the J was still out. Yeah, right.
It's not really a case of watching the spades. Declarer comes to me, I win the trick. That's why I duck the 5 of spades lead. I want him to come to me. Actually, I can't help wondering if East would have taken the trick if declarer had led the J!

As a side issue, let's suppose declarer, instead of leading the 10 of spades, led the 8 of diamonds to the Q at trick 10, followed by a heart, which East and the closed hand are both out of. The hand would've looked like this:

------
J 3
------
9
------ Q 8 7
Q 10 ------
------ ------
Q ------
10 9 6
------
------
------

This has nothing to do with fourth hand, but a lot to do with making that other guy come to you (as much as possible). You'll note that if East plays the Q on that trick, it's all over, bid & made. East must play the 8 or 7, and now declarer, presumably winning with the 10 or 9, will have to play from the 9 (or 10) into East's Q 8 tenace.