I found a website that shows how to make one, but unless you can read German all you can do is look at the pictures.

You wind about 4-5 wraps of braided fastflight serving around a couple of nails about 1 1/2" or so apart, and tie the tag ends with a non-slip knot. Then you fold the loops around the string and serve it into one loop. You have to put tied on nock points inside the loop to keep it from sliding up and down the string.

Maybe that explanation along with the pictures can at least give you an idea what it is.

http://www.compoundbow.de/catfish_loop.htm