At start-up as water enters irrigation system the valve extracts air that is in the system.

The release stage ends as water reaches the inside of the valve raising the
float and closing the main discharge outlet.
At system shutdown the valve prevents pipe collapse by allowing atmospheric air into the system to restore the inside of the system to zero pressure.

The release stage ends as water reaches the inside of the valve. It exerts continuous automatic control, eliminating any residual pocket of ir that might reach the valve, as when this happens, the float drops, partially or totally opening the discharge outlet. It prevents pipe collapse, because pressure decreasing.

The release stage continues to extract air automatically. It permits to water to enter only to the arrest of the system.