There is an arm connected to the valve inside the carb which has a spring holding the valve closed. The diaphram makes contact with the other end of the arm. When the vacuum in the throat of the carb is low enough it pulls on the diaphram, opening the valve and letting fuel in. The higher the RPM, the lower the pressure, the further the valve opens. The pressure it takes to open the valve is the pop off pressure. It can be raised or lowered by lengthening or shortening the spring. Pressure should be between 12 and 18 lbs. Can be tailored to the engine if you know what your doing. Has the same affect as lowering or raising the float in a float bowl type carb.
Gord.