Control Piston
WARNING: This page does not describe the selected car, but rather 9 other vehicles, including the 2019 Ford Transit-350 HD, 2019 Ford Transit-350, 2019 Ford Transit-250, 2019 Ford Transit-150, and 2019 Ford F-550 Super Duty. 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.