The Alma Piston Digital Control Valve works on the principle of "balanced piston".
Piston is always exposed to high upstream pressure from its bottom side, whereas pressure in the piston chamber can be varied with the help of solenoid valves. When pressure in the piston chamber is equal to the high upstream pressure, the top spring acts as a differential force, pushing the piston down on the valve seat and the valve is held tightly-closed.
When pressure in the piston chamber is relieved with the help of solenoid valves, a differential pressure is created across the piston. The high upstream pressure below the piston overcomes the top pressure plus the spring force and pushes the piston up, resulting in valve opening and the flow starts.