The backdoor technique, also known as the six-finger plan, is a strategic move devised by Marvin Latimer and Nakomis Dedmon, and executed by the latter in Big Brother 5. It requires not nominating the intended target to be evicted at the nomination ceremony. Provided the intended target doesn't play in the Power of Veto and/or win, the POV holder would then remove a pawn from the block, and the HOH would place the intended target on the block to be subsequently evicted.
It is a difficult technique to pull off, but if you have a group large enough to carry it out, it can be performed. It requires having 6 players who would agree with the plan, and enough non-HOH and non-nominee houseguests who want to evict the target.
The backdoor was further complicated in Big Brother All Stars when players no longer selected their players, but instead drew names from a bag. If the target plays the POV and loses, then they become the replacement nominee and the backdoor would work out differently.