The leg though is a side effect of the activated root chakra, the first center at the tip of the tailbone. It's out of powering this chakra, this energy center, that makes the leg shoot up.
It is from the root chakra, the base, the 'Earth' of our spine, that we learn to guide the leg up in an upward, vertical direction. By bringing one leg upright against gravity like this, there is a direct use of the will, of putting action out into the world -- literally in this case.
Once the intention becomes strong within, the root, activated, then the leg carries itself up and up, the same way a spring flower shoots through the ground.