SAW dives into its second act in its first minute: Two men in a subterranean TRAINSPOTTING bathroom, chained by their ankles to steel pipes at opposite ends of the room. A dead man lies in a pool of blood on the floor; in his hands, a gun and microcassette player. The men find cassettes in their pockets instructing them on how to escape their plight. One of them has to kill the other…