The mechanism inside the nozzle that controls the shutoff feature is a check valve, positioned at the point where the nozzle spout is attached to the nozzle body. A small hole in the seat of the check valve is connected to a tee leading into two passageways. One passage goes to a diaphragm which, when closed, will trip the mechanism that shuts off the nozzle. The other passage leads to a hole at the tip of the nozzle.
When the fueling process begins, the nozzle spout is well above the liquid level in the vehicle tank. Air enters the hole at the end of the nozzle spout and moves back into the check valve. As long as air continues to enter the hole, the check valve is incapable of generating a vacuum strong enough to trip the control diaphragm. Eventually, however, the liquid level in the vehicle tank will rise to a point where the liquid covers the hole in the end of the nozzle spout.
When this occurs, a strong vacuum is created in the check valve passageways. This vacuum causes the diaphragm to move and trip the mechanism that closes the nozzle.
The principle involved in an automatic nozzle is the Venturi principle.
See also Venturi tube.