In the case of pilot assisted valves, a diaphragm is placed between the solenoid and the main valve opening and it is a variation in pressure at this diaphragm that activates the valve.
When the solenoid is energised, the plunger opens the pilot orifice in the centre of the diaphragm and relieves pressure from the top of the diaphragm to the outlet side of the valve. This results in an unbalanced condition which allows the main line pressure to lift the diaphragm off the main orifice and, thereby, open the valve.
When the solenoid is de-energised, the pilot orifice closes and full line pressure is applied to the top of the diaphragm through a bleed orifice. This provides a seating force that tightly closes the valve. Normally Open pilot assisted valves operate on a similar principle but with opposite effects from the pressure variations.
Pilot Assisted solenoid valves are used to control much greater flow or pressure requirements than direct acting valves. | 
