We've done the figure fours on the arm. Maybe for self-defense, we'll get to learn how to do them on the lower leg. Where does the break (or breaks) occur on the leg?
you want to bony part of your wrist to cut into the narrowest part of the ankle, which is where the damage will occur. some people think that this lock only causes pain but no actual damage, but i promise they are wrong! properly applied, this will put lots of stress on the achilles tendon & hyper-extend the ankle.