Algodoo

The invisible lizard shoes only allow forward movement. The ratchet shoe logic is as follows:

If the shoe is stopped and has a forward force, then lower the hingeConstant to allow the foot to go forward.
If the foot is moving forward but then starts to move backwards, then increase the hinge constant to anchor the foot in place and prevent backwards motion. The blue circle is the shoe and the yellow circle is the anchor pad. The anchor pad automatically follows the shoe. The blue circle must have its pie slice pointing forward in order to work correctly.

Rev A: Added reverse.