This deal from the 2012 Blue Ribbon Pairs was reported to me by 3 different participants! I think the best problem was East's in this variation. East holds: 
AQJ874
8
QJ
KJ97.
Declarer opens 1 on your left and North responds 1NT, forcing. Assume you overcall 2 and hear LHO jump to 4. North now raises to 5. What's that? Usually, a raise to 5 of a major when the opponents have bid a suit, asks for a control there. Indeed, declarer carries on to 6, typically indicating 2nd round control (with first-round control, he should try for 7, by making some other call).
K52
9
A9532
A842
|
|
| |
AQJ874
8
QJ
KJ97
|
|
|
Partner leads the 10, covered by the king and your ace. Declarer plays the 3. Over to you.
With two spade losers, declarer would have passed the invitational 5 bid. So it is insulting of you to try another high spade (maybe some opponents deserve to be insulted, but this was the Blue Ribbon Pairs). Would partner have led the 10 from 109x? Probably so.
What switch makes sense? If declarer has the K that gives him 2 diamonds, 1 club and he would need 9 hearts to make his contract. If that is the case, nothing you do matters.
What if declarer has 8 or fewer hearts? Then, he will need to do something with dummy's diamonds. If declarer has K10x, there isn't much you can do about it. But, what if he has only Kx? A look at the full deal shows what the winning defense is:
Vul: E-W
Dlr: South |
K52
9
A9532
A842
|
|
1096
753
10876
1053
|
|
AQJ874
8
QJ
KJ97
|
| |
3
AKQJ10642
K4
Q6
|
|
Do you see it? If you play anything but a heart, the contract makes. Declarer can play the top diamonds and ruff a third round. Then, he goes to dummy's 9 and plays a 4th diamond to ruff. That sets up the 5th diamond and declarer still has the A to get back to dummy for the 5th diamond.
Your trump shift at trick 2 kills the entry prematurely. Now, declarer can't set up and use dummy's diamonds.
Incidentally, the play of the K at trick two (to knock out dummy's ace), is not a bad shot. Of course, it wouldn't work on this layout, but it is possible to construct a deal where it would be right. So, don't feel badly if you tried the K at trick two; it just wasn't your day.
At one table, West led the 10, ducked all around. West now had to play a club at trick two, but continued spades to allow the contract to make (declarer set up dummy's 5th diamond). Remarkably, a trump shift at trick two (with the K still in dummy) is no good. Declarer can run trumps and catch East in a criss-cross black-suit squeeze. But it's not my style to get into such things in my intermediate-level bridge articles.
