When liquid is pumped through the system at startup, the initial air inside the piping is pushed into the air vent by the pressure of the flow. The float remains in the lower part of the air vent, allowing the valve to remain open and continuously discharge air.
After the initial air venting, liquid flows into the air vent. The float rises with the rising liquid and closes the valve.
If air enters the air vent while it is closed, the liquid level drops and the float lowers, allowing the valve to open and discharge air once more.