Leg pieces are special action blocks that can walk and jump. They used to be used for building characters, but after the introduction of Blocksters are now solely used for animals. Blocksters function the same way but with added behaviors.
Movement[]
Leg pieces and Blocksters both contain code known as "balancing biped." This code was originally written by Martin Magni, and it utilizes two feet objects that will move in order to stabilize the root object.
History[]
Leg pieces have been in Blocksworld since the very earliest version. For a while after they were first added, leg pieces would naturally stumble when pushed in any direction. This was a result of the automatic alignment force not being implemented yet, though the alignment force was activated if they had the "stay" action.
This behavior was also displayed by Blocksters when they were initially added, though they didn't balance as well. Blocksters did have the ability to swim though.
The stumbling was removed some time in 2016 when an automatic alignment force was added.