Playing five-card majors, South opened One Heart and, stretching somewhat, North raised to Three. South went on to game and West led 4Q against Four Hearts. South held off the first round but took his ace when West led 42 to his partner's king. At first sight he had four losers: two spades, a diamond and a club (unless, perhaps, an opponent held #Q,J alone). There was, however, another possibility: he might find West with 2K,x,x or 2Q,x,x.
At trick three, South played off the ace and another club to leave the defenders with an insoluble problem. If West won with his queen, declarer would be able to take a ruffing finesse against East's king; if West allowed his partner to win 2J with his king, South would be able to ruff a club in hand to bring down West's queen and establish 210 for a diamond discard.
In practice, West played low, East won, and led a spade to West's jack. West did his best by leading the thirteenth spade which went to !3, !8 and !J. Then, after two rounds of trumps ending in dummy, a club ruff established South's tenth trick to make the contract.