And When Not To

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

The opening lead allowed declarer a heart trick he wasn't entitled to, but that wasn't what did the defense in. The Q held, of course and declarer followed that by cashing his top spades in the closed hand, then the A of diamonds, followed by the Q, East taking the trick with the K. And the rest was a cakewalk, with -- whoa! That only comes out to 11. There musta been one more defensive error. Lemme see.
Declarer took East's shift to a club with the A, went to dummy with a diamond to finish off the suit, sluffing a club, then cashed the K of spades, sluffing a heart, came to the K of clubs, cashed the A of hearts and the last trick fell to his 7 of clubs, as each defender, who had both been able to guard the third round of clubs, guarded a non-existent threat, West a spade, East a heart.
First things first: East has, or should have, an exact count on the diamonds, and this is perhaps the primary reason for not taking a trick, fourth hand: to deny declarer an entry. East can see that declarer has four diamonds, and that by holding off until round four, he will certainly deny declarer an entry in diamonds, and maybe an entry to dummy altogether. And he should deny his opponent that luxury.
Well, gosh, suppose, um-m-mmmm, declarer has another spade, cashed the A Q to fake me into thinking he was unblocking. Now if I don't take my K of diamonds . . . well, what? If that's the case, if we give declarer one more spade and one less club, declarer has 5 spade winners (on a marked finesse), two hearts, four diamonds, regardless of when East takes his K, and presumably a club. You're not going to beat the contract anyway if declarer has an entry to dummy on a third round of spades. And so would do well to hold off giving him that sure entry by playing the K only on the fourth round. Declarer would then have much trouble and indeed, would be playing the rest of the hand out of the closed hand, down several tricks.
But he still should've been beat. The diamond gaffe only gave him two clubs, four diamonds, two hearts and 3 spades. Couldn't the defense have prevented that 12th trick? Indeed, they could have, obviously, the question then being, couldn't they have seen that? Well, let's see.
The most obvious culprit, of course, has to be West, hanging on to that useless J of spades. But wait! Was it that easy? Here's what West was facing at trick 9, before the lead of the last diamond:

K 10 7
------
7
J
J 9 ------
K 6 J 9 8
------ ------
Q 10 8
------
A 10
------
K 7 6

West must hold onto two spades there or declarer could run the suit. He must hang onto the Q of clubs, or declarer, who sluffed a low club on the last diamond, merely cashes the J, then the K of spades, sluffing a heart and claims. Well, what about East? Well, he must hang onto two clubs, or declarer could make a winner out of the third round 7. So his only safe discard is a heart. And now comes the K of spades! West can handle that lead, of course, but East? He is now squeezed in hearts and clubs. The hand would've looked like this before the lead of the K of spades:

K 10 7
------
------
J
J 9 ------
K J 9
------ ------
Q 10 8
------
A 10
------
K 7

And it is self-evident that East is squeezed on that lead. Declarer sluffs whatever suit East holds onto and claims. My Lord, what a fantastic hand this turned out to be on East's blunder. The basic point still holds. East must hold up till the 4th round of diamonds, and declarer wouldn't have had a chance. He would have no access to the K of spades and no access to the 5th diamond. But in going up, East set up a really fascinating double squeeze, which I confess I didn't see until I looked up what "foolish" discards the defense had made -- and found there weren't any! Not foolish. They did what they could, and declarer took full advantage of that trick 5 foolishness, where East took round 2 of diamonds.
Actually . . . in going back over the hand, I see that West did what he had to do, blanking his K of hearts at trick 9, so as to keep two spades, which he absolutely had to, the club Q to cover the J and that heart. But East played squeezed before he had to, sluffing two clubs on the 7 of diamonds and K of spades, so as to save 3 hearts, one more than necessary. But it didn't matter. Had he sluffed a heart at trick 9, saving two clubs and two hearts, he then would have been genuinely squeezed at trick 10 on the lead of the K of spades.
A wonderfully intriguing hand, thanks to East. You couldn't devises a hand this intriguing if you spend a month of Sundays.