West leads the ♠K. East follows with the 9, and you (South) duck. West next plays the ♠10, and East shows out. What’s your plan?
First, count your tricks. You have one spade, three hearts, two diamonds and a club: seven tricks. Next, count your combined high-card strength: 14 in your hand and 13 in dummy, or 27 total. West opened the bidding, however, so you know he has all the missing high cards (East might have a jack). Therefore, after winning the ♠A, you should take the diamond finesse at trick three because West is marked with the queen for his opening bid. When the finesse succeeds, your trick total is up to eight. But where will the ninth trick come from?
Even though you know West has the ♣K, you don’t have time to play a low club toward dummy’s queen. West will surely grab his ♣K and finish running the spades for down one (four spade tricks and one club for the defense).
Is there any hope? Yes. If you can somehow get West to lead clubs for you, you can take a trick with the ♣Q. How can that be done? West will never cooperate by leading a club voluntarily, of course, but if that’s all he has left in his hand, he’ll have no choice.
Start by cashing all your red-suit winners — two more rounds of diamonds and three rounds of hearts. West can discard a club. Next, exit with a spade, allowing West to finish running his suit. When he’s out of spades, he’ll have nothing left in his hand but clubs, and dummy’s queen will be the game-going trick.
This technique is called an elimination and throw-in.