It's a special type of screen that resists clogging and due to the decline and the "coanda effect" induces solids to slide down the screen to the trough where they end up in two sock filters. The cleaned water (primary filtration of 200 micron screen) falls through to a 55 gallon blue drum which is my moving bed filter with up to 4 cubic feet of plastic media. (I get mine from Allied Aqua which has more surface area than Kaldnes media and is cheaper). All the water from the biofilter and that makes it's way through the socks, ends up in a small side tank (a kind of sump) and is then pumped through a DIY sand and gravel filter for polishing. See my thread on my DIY sand and gravel filter I downsides from "Birdman." The water then makes it's way back to the fish tank via gravity flow as the sand and gravel filter is set up a little higher than the fish tank.
The screen is a used screen that I bought on a koi site (Koiphen). The entire box units are sold for Koi ponds and cost in the neighbor hood of a 1000 US dollars give or take. I bought the screen for 200 U.S. dollars and two Uniseals, some 3 inch PVC, the tub, and two stainless steel screws probably totaled less than $20.00.
Here is a video of a commercial unit: