The lead of one of a sequence of high cards towards a void. If the player in second seat does not cover, you allow the lead to run; if there is a cover, you ruff and later return to cash the established winners.
For example:
As South, you lead the king of this side-suit from dummy. If East plays his ace, you ruff, thus establishing the Q-J-10 as winners without losing the lead. If East plays low, you discard a loser.
If West held the ace, the ruffing finesse would fail, though you might still be able to discard a loser and succeed in setting up dummy’s winners.