Let’s consider E as an electromotive force in one branch and I be the current produced by E in second branch

Reciprocity Theorem states that when E from the first branch is moved to the second branch, it will produce the same current I in the first branch as produced before. In this case, E will be replaced by a short circuit on the first branch. It states that electromotive force and current produces by the same are mutually transferrable. The ratio of the same which is E/I is termed as the transfer resistance which is also known as impedance resistance in AC systems.