This is one of many examples of dynamic response feet, which are indicated for people whose gait patterns generate enough energy to be worth storing. For this reason, some prosthetists call them "energy storing feet." They incorporate elastic keel structures that absorb energy during midstance and terminal stance, and then "release" it during preswing and initial swing. Many types are available, including the College Park Foot (pictured here), the Otto Bock Dynamic Foot, the Flex Foot, the Seattle Foot, the Quantum Foot, and the Carbon Copy II. The durability of some new materials is not fully tested.
For an excellent summary of dynamic response feet, see Edelstein, J.E. (1988). Prosthetic feet: State of the art. Physical Therapy, 68, 1874-1881.