Pressure servo assisted or pilot assisted valves work with only pressure assistance or pressure differential there is no mechanical link between the sealing diaphragm or piston to the controlling armature assembly. In this case the diaphragm or piston is allowed to "float" and be controlled by remote small servo pilotpilot operated solenoid valve circuit that utilises and controls the valve differential pressure (the pressure difference between inlet and outlet pressure) to move the valve into the open / closed positions.
The picture on the right / below shows a 2/2 way normally closed pilot valve in this case about 11/2 or 2" thread with typically a smaller solenoid coil mounted over the outlet port as no additional force is required to mechanically lift an attached diaphragm.
The principle here is much the same as the assisted lift pressure valve mentioned above, in that there are pilot holes utilised to control the pressure above the diaphragm. Just in this case there is one small inlet pilot hole to one side of the diaphragm and one larger outlet pilot leading from above the diaphragm under the controlling solenoid and armature assembly through the valve body and then out to the outlet port.
So for example a 2/2 way normally closed pilot / servo assisted or floating diaphragm or piston solenoid valve will when energised release the pressure above the seal down stream through the outlet port so the inlet pressure can lift the seal/piston into the open position. When electrical power is removed the pilot circuit closes, pressure above the seal quickly increases and pushes the piston/ diaphragm closed.