In the Fly Fishing 101 section I read the following:
"Adding a "riffle hitch" to the head of the fly will help keep it riding on the surface. A riffle-hitch is simply a half-hitch that is secured around the head of the fly, usually about 1/4 inch behind the eye of the hook."

I don't understand what he means. Can anyone explain exactly what a riffle-hitch is and how it is done?

Thanks,
DieKatze