PPT Slide
First take an arbitrary pure strategy of player II. Each strategic pair in its column stands for a different strategy of player I, among them his sgp strategy.
A pure strategy that differs only in irrelevant nodes from player I𠏋 sgp strategy must produce exactly the same payoff. To differ in relevant nodes means to lead player II from the path that by definition was his best response to all strategies of the opponent. This can never increase his payoff.