The backdoor technique is a strategic move. It required not nominating the intended target to be evicted at the nomination ceremony. Provided the intended target did not play for the Power of Veto, 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.
In Week 1, Harold and Leonard both decided to put Duncan up as a replacement for Lightning. Duncan was successfully evicted as he received five votes.
In Week 4,