What are you referring to when you say downdraft protein skimmer?

I consider a downdraft protein skimmer one that is fed at the top, water flows downward and the output (for figurative reasons) is at the bottom.
Ok my understanding is 3 kinds of skimmers

1. A counter current uses small pump and airstones 
2. A Venturi that uses larger pump and a Venturi valve restrictor and small air intake 
3. A downdraft or ETS environmental tower scrubber. 

This has a TALL like 5 ft x 2" "tower" filled with bioballs, the water gets pumped into the top of this column under relatively high pressure through a fairly small 5/16 or 3/8 piece of plumbing fitting kind of like a tapered fire hose nozzle. At the top of this 2in x 5ft tower there is a hole about 3/8in that allows air to be "down drafted" into this bioball column The turbulated, air induced, foamed water then crashes into a small rectangular box about 8x8x12 inches the foam then drifts over to a LARGE like 8in acrylic reaction foam collection, accumulation riser, much like a regular protein skimmer. Bubbles burst foam rises, and overflows into a collection area like a regular skimmer.

 This works very well as there is virtually no restriction to a very high flow rate. Mine processes 1200gph for the skimmer and another main pump 1200 gph for the main

Simplified Reefkeeping