A normal finesse is the lead of a card towards a tenace position, with the intention of playing the lower of the top two cards if the missing card is not forthcoming.
Now, suppose you know that the missing card is over the tenace, you may take the finesse ‘backwards’. For example:
Needing three tricks in the suit, as South you would normally play a small card from dummy towards the K-J, playing East for the queen. However, if you judge that West holds the queen (perhaps he has opened the bidding and shown up with only 10 points in the other suits), you may lead the jack from hand. If West covers with the queen, North’s ace wins and you play back towards the K-9 finessing East for the ten.