I would think this device would really help for people who have a lot of neighbors living above grade from them. If a sewer main happened to clog up the poop water having no where to go could divert and travel in reverse direction inside your house. That would be a nightmare.
Those do not work!! A simple back flow trickle of sewer water will not trip that flapper. You need a sudden full pipe slug of water to trip it. Have a whole area in W. Toronto that has those valves and none worked..Flooded basements galore July 16 and Aug 17.