There are standard elimination situations, where the declarer has an exit, and defenders have to play the suit, where the declarer needs help.
This hand is a little bit different. Let's see, what had happened?
This hand we played in a team match in Hungary.
The opening lead was the King of clubs, and the Ace of clubs came in the next trick, as a little help, despite of we don't have a loser to throw away.
I ruffed the 2nd round of clubs in my hand and played a heart to the dummy. West won it with the Ace, and played small spade.
Now this is an eliminated situation, if we cash the King of hearts, and the Jack too, because West had singleton Ace of hearts.
Now, we can cash our club trick, (East follows it) , discarding the 4th diamond in the hand and we can cash 3 rounds of spades, discarding the 4th diamond in the dummy, followed by both opponents.
We come to the 4 card endplay like this:
What do we know from West's hand?
He doesn't have 4 cards in ♠, because he had a chance to bid 1♠ or double, but he didn't. So he has 3 spades, he has singleton in heart, and exactly 6 cards in clubs as we saw. That means, that West has exactly 3 diamonds, and the King is almost surely at him.
Now, we can eliminate West, if he has also the Jack of diamonds, playing the ten of diamonds from the hand, forcing him to win this trick, and either he should play diamond from the King or a double void, to give us the contract.
So we play the ten of diamonds from the hand,but West doesn't cover it. Now, either he hasn't the Jack of diamonds or he was very brave to serve INMEDIATELY small from King and Jack of diamonds, which is an obligatory card in high level bridge anyway, otherwise West has no chance to take 2 diamond tricks, whis is necessary to defeat the contract. Because, if the declarer has the Queen of diamonds, now West is eliminated, if East has the Queen of diamonds, West must put small diamond to the 1st trick, otherwise East will be eliminated, with the dobleton Queen of diamonds in trick 2.
Anyway, I decided so, that West doesn't have both diamond honors, and my lucky day came now, because I had to put the Ace of diamonds and play the 2nd round, hoping, that East doesn't discard his Jack of diamonds into this trick, and he will be eliminated, when I play the 2nd round of diamond from the dummy.
If he doesn't discard it, I can play low from my Queen, and West cannot do anything. So the only way is for East to put the Jack of diamonds to the Ace in the 1st trick, hoping, that West can take the 2nd and 3rd rounds of diamonds.
Anyway, East was in a hopeless situation, because he had Jack and nine of diamonds, and he couldn't come away from the 2nd trick of diamonds, because West had K83. What a lucky hand, still we had to overrule our 1st decision correctly.
On the other side? Declarer played Ace of diamonds and a diamond from the dummy, followed by East's Jack, just made.
Boring hand, isn't it?
The complete had looked like this: