 The cup on the muzzle of the barrel provides a large, movable surface for the gas to push against, as it exerts force equally in all directions.As the outer shroud is fixed to the main frame of the gun, and only the barrel is movable, the pressure forces the cup and barrel to the rear, acting exactly as a piston in a cylinder.As the barrel cycles to the rear, the cup passes the perforations in the outer shroud, opening an escape path for the gases, immediately lowering the pressure, both reducing the harsh impact when the barrel reaches full recoil and preventing the remaining pressure from the gases from acting as a "spring" and slowing the barrel when it begins to travel forwards again (which would only slow the rate of fire).