It's called an epoxy mixing nozzle.
You simply cut the tip off to permit 1/4" of flow, and insert the thing somewhere into your beer line. It has a spiral innard that works like a coiled tube - it increases the length of tube the beer has to travel from intake to faucet spout. That helps bring down the foaminess; it gets you a better pour.
On Northern Homebrewer, the guy who posted it had it in the middle of the beer line between the tap and the faucet. I have mine as part of the tap itself, inside the keg.
EDIT: The final piece of my puzzle has arrived. To this point I've been using copper tubing in the tap. It's fine for the short-term, but over the long haul beer reacts with the copper and it's not good for you. The replacement stainless 1/4" tubing is arriving today. I will bend two pieces, one for my portable tap and one for this new tap.