A statement from Ponssard and Zamir "Zero-sum sequential games with incomplete information" at the bottom of the first page reads
AM's result is for zero-sum two-person games in which the two players move simultaneously. This includes of course games in which the moves are made sequentially: Player I moves first, player II is informed of player I's move and then makes his own move. (Such a game can be looked at as a simultaneous game after redefining the pure strategies of player II.)
I am curious about the last statement in parentheses. How does one "redefine" the strategies of the follower to make the game simultaneous move?