There's a ping-pong ball inside the white pvc pipe that you see sticking out. What it does is when the wc gets full, the ping-pong ball pushes up and shut off the air therefore shutting the air flow through the skimmer. The skimmer would still be running but it won't be skimming. Not sure if you getting the picture. But anyway, that's what it does. I'm not sure that this will work with any skimmer, but definitely will work with beckett skimmer.