Well, As far as I can tell their are at least 2 things I can think of. 1) Her feet weigh less than ordinary feet. Better mass to horsepower ratio. Ask any car or bike racer about that, it’s why things like NASCAR very strictly limit certain parameters. 2) Like tiki said, springs for feet. Rather than using the normal muscular contraction/expansion mechanic for performing work, less energy is being used to compress the spring then release it at the proper angle.

All that having been said, the fact that medical science can produce a synthetic limb with that alows a person to compete at a professional level of athletics is fantastic. A real step ahead for normal amputees, everyday joes and sallies who lost their limbs and simply want to go back to normal routines.