Control Piston
WARNING: This page is about a different car, the 2012 Ford Pickup, 2012 Ford F550 Super Duty, 2012 Ford F450 Super Duty, and 2012 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
The control piston uses its large surface area on top as a downward force to overcome an upward force created by the smaller surface area in the high pressure chamber. The control piston also keeps the nozzle needle in the closed position when the piezo actuator is not energized.