Basically, the Calfo overflow consists of a full length overflow box attached to the back of the tank, with a shelf on the front. The idea is that by skimming the entire length of the tank, you do a much better job of exporting pollutants. The shelf can be used to camoflage the overflow with corals, mushrooms, xenia, etc.

Another idea that he has is that reef tanks need more turbulence rather than just regular wave-type flow. He achieves this through multiple water returns all around the perimeter of the tank instead of powerheads. You can go to http://www.wetwebmedia.com to read more about both ideas.

For reference, here is a sketch of what I'm toying with for my next tank. It includes both the Calfo overflow idea and the increased turbulence theory.