[Delphinus]

The skimmer essentially needs to be sealed (except for the drain tube) in order for this to work. So the air that gets sucked in by the beckett, it has to have somewhere to go, so it exits through the drain tube into the waste collection cup (the waste collector has to be vented). Once the ping pong ball closes off the drain tube, from the collection cup, the air can't get through to that vent anymore and this basically shuts down the skimmer. It actually doesn't take much of a blockage to stop a beckett from skimming. HTH