The forces required to open and close the nozzle needle cannot be generated by the solenoid valve on its own. The nozzle needle is therefore indirectly triggered via a hydraulic force-amplification system.
With the solenoid valve closed, the complete chamber volume and the rail are at the same pressure. The nozzle needle is forced against its seat by a spring.
When the solenoid valve opens, fuel flows from the valve control cavity and into the fuel return.The feed throttle prevents complete pressure equalization, and the pressure in the cavity drops. The excess pressure in the chamber volume overcomes the spring force and lifts the needle so that injection can start.
The solenoid valve is no longer energized and closes the opening to the fuel return. The force applied to the control plunger increases along with the increasing pressure in the valve control cavity. The needle closes and injection stops.