Electro-Hydraulic Injector Amplifier Piston
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2006 Ford RV Cutaway, 2006 Ford Pickup, 2006 Ford F550 Super Duty, 2006 Ford F450 Super Duty, and 2006 Ford Econoline. However, it is still accessible from the selected car via links, so may be relevant.
The high pressure oil flows from the oil rails into an electro-hydraulic injector amplifier piston located in the injector. Oil entry and exit to and from the amplifier piston is controlled by a coil-operated spool valve.