There was some discussion recently on them here.
http://www.bikeforums.net/showthread...edal-extenders
I believe the idea is to have your pedal stroke lined up so it is straight throughout. In my case the fitter suggested them to alleviate hotspots i was getting on the out side of my foot. Without them my knees where flaring out at the top of the stroke putting additional pressure on the outside of my foot. with them my knee stays close to center and pressure on the foot is even.