This is the idea that if an opponent makes a particular play, especially dropping a high card, this is more likely to be a forced (ie restricted) play rather than a chosen play.
Suppose you cash the ace and cross to the queen, picking up the jack (or ten) from West on the second round. If the jack (or ten) is a forced play from a doubleton honour, you want to finesse the nine on the third round.
Finessing is normally correct because with J-10-x West might equally have played the other honour on the second round.