The Herbies have an overflow box with TWO drainlines on each overflow. Once drain line is just a bulkhead out the bottom of the box, call it drain A. Drain B is in the same box next to Drain A, and is also a bulkhead on the bottom of the box, but Drain B has a pipe on it so that it is several inches taller than Drain A. Drain A has a gate valve on it to constrict the flow so that the water in the box remains a couple inches above drain A's bulkhead, so there is NO air in Drain A drain line. Drain B doesn't have any valves on it. Now, eventually Drain A will clog or partially clog which will back the water up so it raises in the box. When it gets high enough the water will start going down Drain B along with a bunch of air as it "sucks". It will happily run this way until there is a point in which Drain B also gets clogged, but this won't happen because Drain B will be really noisy. This is your alert that Drain A has clogged, and needs maintenance.
Now, how's that for confusing?
Here's a pic of my tentative plumbing including the Herbie overflow. Drain A would be the green line on the right, and Drain B would be the green line on the left. On my line A there is a gate valve (the higher black thingy), and a union (the lower black thingy).
Here's a link to the original Herbie thread on Reef Central:
http://www.reefcentral.com/forums/sh...hreadid=344892