What part causes the disc brake caliper piston to retract when the brakes are released?

The part that causes the disc caliper piston to retract when the brakes are released is the square-cut O-ring.

The square cut seal is the most important part of a disc brake caliper, for keeping the brake behind the piston so that when you step on the brake pedal, it releases a pressure that applied to the piston which in return applies the pad to the rotor.