This is the one I'm using,
http://www.jlaquatics.com/phpstore/s..._ID=ps-pmpw410
There's a larger version made by PM as well, as well as a number of other brands that sell the same thing.
The collection cup drains into this thing. There is an "exhaust" port that has a ping pong ball that floats. If the waste collector fills up, the ping pong ball floats and pushes up against the exhaust port. (It's basically a low-tech "foot valve.") Then air can't escape out, which puts back pressure on the skimmer, and the skimmer can't produce foam against the backpressure. So although the skimmer is still "running" per se, it's effectively "shut down" until you empty the waste collector.