As you seem to be still online
Vehicles fitted with disc brakes all around need a mechanical handbrake the same as all others. This is easy for drum brakes - a simple lever/cam works quite well. On disc brakes this isn't as easy and is accomplished by a lever/cam working on one of the inner pads (nothing special) however as soon as you apply the handbrake the piston behind the opposite pad is pushed back in and there is not an effective braking force.
To overcome this the rear pistons must work on a ratchet mechanism to ensure that as the pad wears the ratchet takes up the 'play', rotating the inner piston out which ensures that when the handbrake is applied the whole piston assembly is only allowed to move a short distance before stopping (as it has reached the maximum travel back within the hydraulic bore).
The mechanism is basically a cylinder that rotates (friction keeping it in place) out away from the hydraulic piston..
When you need to replace the rear pads you must first ensure the rear pistons are fully retracted - to do this you must wind the inner piston back in relative to the hydraulic piston. It is a simple rotary action but the nature and location of brakes pistons usually it requires a bit of a push. It can be a pig to do even with the right tool.
The rear piston faces are keyed to take a tool to do this - commonly known as a Wind Back Tool. They can be as cheap as £6.
Hope I've explained it well?
* Just remembered there is a ratchet to stop the shoes going back in on drum brakes but this works as part of the main braking mechanism as well.
** Some vehicles have a small drum brake fitted just for the handbrake - not familiar with this!