One of the most rewarding defensive maneuvers is achieving an uppercut. You essentially manufacture a trump trick out of thin air.
In a club game, partnering a good player (and frequent teammate) for the first time, we have decided to play upside-down count and attitude, but nothing beyond that. Still, we are doing well ( we would finish second, with 57%) when the opponents find themselves in a nice 3C partscore.
I lead partner's suit and the play proceeds up this point (hit Next to see the play) when the cards are as follows:
From partner's point of view, I could have either the King of hearts or the Jack of clubs to defeat this contract. If I have the King of hearts, he needs to lead a heart before declarer can discard his heart loser on the good spade. If, on the other hand, I have the Jack of clubs, then an upper-cut is the ticket to beating 3C. My signal needs to tell him which line to take.
Discouraging hearts, perhaps with the 10 of hearts would have lead partner to finding the uppercut. He leads a diamond and if declarer ruffs with the Ace, my Jack is good ...
(Yes, declarer misplayed this. If he pulls trump before playing spades, there is no upper-cut. Also, he finessed spades into the wrong hand. If he finesses the spade into my hand, his hearts would be safe even if I held the Ace of hearts. But then, this ain't the Bermuda Bowl.)