The pressure drop across the metering jets creates the "fuel metering force" on the fuel diaphragm which acts to open the servo valve until a balance is reached with the air metering force.
The pressure drop across the metering jets creates the "fuel metering force" on the fuel diaphragm. This force acts to open the servo valve until a balance is reached with the combined force from the un-metered fuel pressure and the air metering force.