S/All		♠	K 3 2
		♥	9 8 6 5
		♦	A 2
		♣	A 7 6 5
		♠	7 6 4
		♥	A K J 10 7 4
		♦	K Q
		♣	K 4

West leads the ♦J. Declarer wins and plays the ♥A. Alas, East throws a diamond.
How should declarer play?

Solution

Declarer is certain to lose a trick to West's ♥Q and is in danger of losing three spade tricks if East has the ♠A (if the ♠A is with West, the contract is on ice).
Declarer has escaped the spade lead. This is fortunate, since it gives him a small extra chance in case the ♠A is with East. If East has four clubs as well, declarer can place him in the lead with that card – provided he has eliminated the diamond suit first.
If so, the layout is for instance (the point is East having four clubs, the rest is immaterial):

S/All		♠	K 3 2
		♥	9 8 6 5
		♦	A 2
		♣	A 7 6 5
♠	J 9 5			♠	A Q 10 8
♥	Q 3 2			♥	-
♦	J 10 9 8			♦	7 6 5 4 3
♣	Q 10 2			♣	J 9 8 3
		♠	7 6 4
		♥	A K J 10 7 4
		♦	K Q
		♣	K 4

After the ♥A declarer plays the ♥K, the ♣K and the ♣A. Next he ruffs a club. If EW follow suit all the way, declarer's chances of success have increased. He crosses to dummy's ♦A (!) and plays the fourth club. If East has it (he follows suit), South discards a spade.
It doesn't help West to ruff: he would ruff one of South's losers with the master trump.
So East is on lead and has to either allow declarer to score the ♠K or play a diamond, giving declarer a ruff and discard (which eliminates one of declarer's spade losers as well: South pitches a spade, dummy ruffs).

It will be clear by now: declarer must have won the lead in hand. If he hasn't, he cannot – after having ruffed the third club – enter dummy in order to play the fourth club!

On a different layout in clubs, declarer can still play for the ♠A to be with West. The throw-in offers an extra chance.