More often than not, declarer is confronted by situations where he must judge which of several approaches is more likely to work out best.
Consider this deal where South is in six hearts and has two potential losers – a diamond and a club. Since there are no finesses or discards available, the only hope is to try for an endplay.
So declarer takes the diamond with the king, draws trumps and cashes the A-K-Q of spades and the ace of diamonds, West discarding a low club. The position now is:
The critical question is which opponent has the king of clubs. If declarer thinks it’s West, he should play the ace and another club, compelling West to win and yield a ruff-and-discard. If he thinks it’s East, he should lead a diamond, forcing East to win and lead a club from the king or concede a ruff-and-discard.
South can’t be certain, but he should reason that West is more likely to have the club king. This is because East has thus far shown up with six diamonds, three spades and one trump, leaving him with at most three clubs. West therefore started with six or seven clubs.
Since West was dealt at least twice as many clubs as East, he is much more likely to have the king. So South plays the ace and another club, forcing West to win and yield a fatal ruff-and-discard.