Hydraulic Coupler
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2016 Ford Transit-350 HD, 2016 Ford Transit-350, 2016 Ford Transit-250, 2016 Ford Transit-150, and 2016 Ford F-550 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
The hydraulic coupler transfers the longitudinal movement from the piezo actuator to the fuel injector control valve. It also acts as a seal preventing fuel from entering the piezo actuator device.