from canon EOS magazine site: Image Stabiliser (IS) dramatically reduces image blur caused by camera shake. Gyro sensors detect unwanted vibrations, triggering the corresponding movement of a correcting lens group perpendicular to the optical axis. This alters the light path, returning the image to its correct position on the sensor or film plane. The 3 stop advantage means that photographers normally shooting a 105mm lens handheld at 1/125 sec can obtain a shake free result with a shutter speed as low as 1/15 sec, for vastly extended options in low light conditions.