Its second-generation Ankle Mimicking Prosthetic Foot (AMP-Foot 2.0) uses a pair of force sensors to determine the leg's relative position and let an actuator build energy when the foot bends, locking the power away to use only when the owner pushes off.
ENGADGET: AMP-Foot 2.0 prosthesis gives the power of real feet, keeps a light step (video)