This rule can only be used backwards on a goal line to prove the goal line by contradiction (i.e. assume the negation of the goal formula and prove that you can reach a contradiction (bottom) inside a box).
To use it backwards:

1. select the goal line you wish to prove by contradiction and click on Proof By Contradiction.
2. a new box will then be added to your proof which assumes the negation of the goal formula and has a conclusion of bottom.