That lever directs the outflow of the contents of the glass cylinder. It operates an internal valve that allows the contents of the glass cylinder to flow either to the hose for delivery--OR-- it allows the contents to return to the underground tank.
The cylinder is filled by the pumping action and if this lever is in the wrong position, the contents simply return to the underground tank. In the correct position, the contents remain in the glass cylinder, awaiting delivery out the delivery hose.
Hope that clarifies it.
