The film starts with Seth in the pendulum trap. It is revealed Seth is a murderer who was released early from prison due to a technicality. Seth is told by Jigsaw's puppet that if he wants to live he has to destroy the things he has used to kill-his hands.
The trap is later revealed to be unwinnable, but Seth is made to believe that the aim of the trap is for him to push buttons inside of two devices which would crush both of his hands before a bladed pendulum (intended to slice him in half) descends upon him.
Seth had in fact, crushed his hands, but as the trap is unwinnable, the Pendulum does not stop. From the side of the room, somebody watches Seth getting sliced in half from a Peep hole.