rear brake question

[Eastlmark]

Can anyone explain how the rear piston in the caliper releases the pressure on the pads when we take our foot off the brake? I understand applying pressure to the master cylinder pumps up the piston when braking but what makes the piston pop back in when not braking? Given we have to wind the piston in when changing pads and that the winding action cannot occur in normal use due to the pin in the back of the pad.