The keel's length determines the timing of heel rise.

As soon as the GRF passes in front of the keel, it exerts a "toe extension" moment on the more flexible anterior portion of the prosthetic foot.

Because the floor stabilizes the foot's anterior portion, the extension moment "breaks the toe." The foot bends at the end of the solid keel, permitting the shank to incline forward, and the heel to rise.