Well, it's only matter of inertia forces. If you start breaking - no matter if you use hand- or footbrake - the front suspension settles down as the inertia forces push the car (including you inside) to the front (yes, at the same time the rear suspension elevates a bit). If you release the brake the inertia forces disappear and the car returns to the normal position - you can feel this movement as sinking of the back.