When pump trips, Water column separation creates partial vacuum and at that juncture air inlet poppet opens due to actionof spring. The air gets entrapped in the pipe line. With reversal of water column. entrapped air gets compressed causing the outlet to open and air escapes through the outlet adjustable plug. The energy of reversing water column thus facilitates entry of water in the pipe line enabling the float to rise and press on the orifice preventing leakage of water.