A small accumulation of water exerts pressure on the check valve diaphragm. This action opens the sealing lip of the membrane upwards and releases the waste water flow. When the wastewater flow stops, the sealing lip lowers again and closes the non-return valve. In the event of a backflow of water, the rubber membrane is filled and pressed against the pipe wall. This function also makes it impossible for the gas to move to prevent unpleasant odours from escaping from the ducts.