Source: It Pays To Give Cards A Chance By OSWALD JACOBY And ALAN SONTAG
Alan: In our current Friday articles, we have been showing how to play certain card combinations involv-ing one particular suit. How about a few articles to illustrate?
OSWALD: Here is an example of a play known as the obligatory finesse. Needless to say there is no one holding a gun to declarer’s head and forcing him to take it. It simply is a play that can win for you and can’t cost you a trick if you take it.
Vulnerable North/South; Dealer North
Opening lead: 
2
ALAN: South finds himself in four spades. Maybe North shouldn’t have bid three. Maybe South shouldn’t have bid four. It doesn’t matter. That’s the contract he has to play.
OSWALD: East takes his ace and queen of hearts. Then he shifts to the jack of diamonds.
ALAN: South has lost two tricks and expects to lose two more. He wins the diamond in his own hand and leads a low trump. West’s 10 loses to dummy’s queen and the deuce of trumps is led back.
OSWALD: East plays the nine and now comes the obligatory finesse. South ducks. West has to play his ace and South gets away with just one trump loser. Luck, of course, but he had given the cards a chance.