back in high school we used to do plyometrics on easy days up a short steep hill. this included the usual stuff like bounds, skips, high knees, butt kicks, and backwards running (back pedals, is what we called them). It was pretty silly, but incredibly painful. You could debate all day about the physiological benefits and neuromuscular jazz and blah blah blah. but in short, its just a thing to challenge different muscle groups. pretty sweet. sounds silly to be doing hill repeats backwards all day though