Wow! Interesting technical coversation. Thanks everyone for the read!
Having come originally from a TKD background myself that did not have any "real" lows kicks (I was always told that if you can kick high, you can kick low, which is BS in my opinion) and then moving to a Kyokushin derivative style for the last 15 years and also having trained for a short while with two guys who trained in MT in Thailand (years ago) it was quite an eye opener for the differences and variations in the kicks.
A back condition and a herniated disc made me appreciate a variation of the low kick that was mentioned above with the downward "chopping" motion. (The severe torquing of MT started to cause me problems.) I have seen Ishi of Seido Karate due this kick in an older video, but it seems to be an amalgam of the MT and Kyokushin derived kicks. It is a bit slower and if done on-axis, facing your opponent, presents problems when face punches are allowed.
However, performing it would come from that traditional plant of the Kyokushin player with the supporting foot flat on the ground and not pivoting (in real application you would be on the balls of your feet with slight pivoting motions of the foot, but this is necessary for training purposes here) so that the kicking leg is whipped in a circular fashion over the hip joint, not turned inward as a hinged door, impacting with the flat of the shin bone about 3-6 inches above the ankle. This requires a slight turn and bend of the torso and your body weight being directed loosely in the direction of the kick so that you transition you mass into the kick, thus one needs to be wary of face punches.
In this case, the impact zone is on the equator of the opponent's thigh with better contact made along the outer edge of the leg about 50 percent of the way between the hip joint and the knee where the muscularture is less dense than on the front of the quad and where it's occasionally possible to hit the nerves there to add a little "umph" to the kick. In sparring this does less to pain the opponent noting the presence of adrenaline, but does give a nice dead-leg...and in some cases can collapse an opponent not used to the contact on that limb.
Generally, this type of kick comes into its own from close in and to the side of your opponent...especially if grabs are allowed.
-Brad Burklund