The Squeeze-Trim-Endplay: Hand
This hand was played by Joe Silver and reported by Don Kersey, in his 1990 Bridge World article analyzing this type of play (which he called a one-threat squeeze).
Lefty opened 1, your partner doubled, Righty bid 3, and you finished the auction with a call of 4. The opening lead is the Q.
You win in dummy and lead a diamond. You would like to ruff some diamonds. Righty pops the A and leads a trump. Lefty wins the ace and leads another trump.
You have to negotiate ruffing a diamond and drawing trump, but once that is done, the contract pretty much depends on finding the J. You can place the A with Lefty, but if there is anything in the bidding or play to indicate who has the J, I don't see it.
Well, anyway, to get some work done, you play your high club, pitching a heart, and ruff a club. To your surprise, Lefty shows out. (All of your trumps are high, so Lefty could not overruff.) Now you ruff your diamond. You still need to get back to your hand, so you ruff another club, Lefty underruffing.
I think that the logical play at this point is to lead a heart to the queen, then finesse the jack of hearts on the way back. This finesse either wins or loses, and then you go on to the next hand and never realize the hand was a potential squeeze-trim-endplay. You cannot avoid the guess of who has the heart jack, but you can play the hand to make if Lefty has it.
Furthermore, I think the play suggests that Lefty has the jack of hearts. First, if Lefty has the last spade and is the one with the 5-card diamond suit, then Lefty has only 3 hearts and it is 50-50 who has the jack. But Righty could have the last spade, or Righty could have the 5-card diamond suit. Or, if Righty went to the three level with only 4 diamonds, Righty might have been motivated by a singleton heart. If Lefty has a 4-card heart suit, Lefty probably has the jack.
So you can finesse for the jack of hearts, if you want. The point is, do you see your other choice?
Actually, Lefty's underruff was a sign that he was being squeezed. The situation is this:
You now know that Lefty started with 3 spades, 2 clubs, and 4 or 5 diamonds. That means he now has 3 hearts and two diamonds, or 4 hearts and one diamond. In either case, it is safe to play a trump, because you have control of the diamonds. On your trump, Lefty discards a diamond. Now it is actually safe to play the K of diamonds. Lefty follows, so Lefty is started with 3 hearts and still has them.
It's your guess -- heart to the queen and finesse on the way back, or lead your king of hearts and let the heart return ride around to your 10. Silver played for the latter, successfully executing a squeeze-trim-endplay.