Consider a dynamic game of imperfect information, G.  Based on G, construct a game, PG, which has the same strategies, payoffs, and order of moves as G except PG is a game of perfect information (every player in PG is aware of the strategies chosen by those players who moved before).
Imagine a game called G where players have imperfect knowledge about each other's moves. Now create another game, PG, that has the same rules as G but with perfect information: every player knows what everyone else has done.