Head to tracko for a brief explanation. Study the photo and you'll understand how the cog works. Basically, you tighten the black cylindrical piece onto your hub, and the three cogs become conveniently interchangeable. I think I'll pump some iron and polish my chain whip before I drop $110 for convienence. None-the-less, it is a terrific design. I wish that the bicycle industry would recognize this technology and standardize the design, so it becomes easier to interchange brands.