It's a rubber/metal ring around the flashlight body, approx. 1 inch in front of the tail switch, so you can hold your torch like a cigar between 2 fingers. comes stock on several eagletacs and there are also optional for solarforce, surefire and so on available.
Surefire even has a manual for it that comes with some of there combat grip lights...does a rather nice job of explaining the different method's, etc. I'll try to get a scan of the manual up one of these days.