Very nice clip, Ray.
However, I personally do not teach the low kick to the opponent's lower
leg using your instep [The arched middle part of the human foot between the toes and the ankle]_
It is 'generally' OK to use the instep to strike at the knee joint and outside and inside thigh of an opponent because there will be some 'flexion' of the target upon impact as opposed to more of a 'rigid' mass when slamming closer down to the ground against the legs of a heavy opponent.
I teach striking the opponent's lower leg using the shin bone above the ankle and up in a more distance committed attack.
The bones can be easily conditioned, the joints much less so.
If you break a bone, you will go into shock within seconds.
Striking with your instep against the 'planted' lower legs of a heavy opponent_will place lots of stress upon the ankle joint that can result in bad sprains or breaks...and your 'fight' is over.
If you watch a soccer game, you [said generally] will quickly realize that soccer players live in 'low kicks land' ...with extremely conditioned legs used to the constant impacts.
From my personal experience as a soccer player...I did develop very powerful legs...yet ankle sprains and near breaks were common.