assuming you have removed the rear caliper from the hub and it is now only connected via the brake pipe and the handbrake cable, do this:

inside the car, let the handbrake off
this will move the pins back to their original stops (although if the handbrake is on, the rear caliper shouldn't be able to come off the disc)
open the fluid resivour to stop any fluid backing up and stopping the piston from retracting
you need to have pressure on the piston while you turn, hence the windback tool is required for some jobs
the piston is pushed out by fluid pressure
the handbrake relies on a thread inside the piston that turns as you apply the handbrake
this also accounts for the autoadjustment
and this is what you need to overcome to get the piston back
remember also that there are 4 grooves in the piston that need to line up (one of them in anycase) with the little pin that sticks out of the middle of the pad

once you have spun it back and lined the pins up, you can put the pads back in
once everythign is done, turn the ignition on so that teh abs pump primes, then use the foot pedal to push the pistons out (on both sides) until they stop the disc from turning
once the piston is out in it's proper place, then use the handbrake to tight it up and set itself

once the piston starts to go back, it will be easy to keep it going backwards