Counterweight for Thrust Kicks
I've been learning a few leaning thrust kicks, mainly we've been practicing a front thrust kick and a stepping side kick.
Its not that I don't know how to do them, I'm no master but I think I have the lean down its the physics that confuses me.
To do the kicks you shift the weight essentially in the opposite direction of the kick and then thrust your leg straight through the target.
My query is how can leaning back give you more force going forward? or leaning right give you more force going left, because it is more force than a snap kick there is no doubt about that.
does the lean just give you more space to thrust? or is there some weird counter physics thing going on?
I'm not 100% sure, but I think that leaning back will make your head harder to hit as it's further away, and you might get a but more reach, depending on your hips. Take that with a grain of salt as I'm no expert. I'm sure that someone more informed will provide a better answer.
I had a couple drinks when I wrote that last night, let me give a better answer. You're thrusting by throwing your hips forward, if you're leaning forward you can't push the hips through which is where the power is coming from. Otherwise it's not a thrust kick.
Your big problem here seems to be that you are looking at these kicks as a static system. You're forgetting to take into account momentum, as the whole body delivering the kick is moving in the direction of the kick. As huge mentioned, the first purpose of the torso lean is not to add more power to the kick but to make your head a harder target. The second is to aid in recovery.
In a static picture it looks like a person leans back when they kick, but what you should actually be doing is throwing your hips forward at the end of the kick, so you get good power and don't just flick your lower leg into the target. The hip thrust is what gives a front kick its thrusting quality rather than an upward swinging kick.
For the side kick, its to achieve the proper body alignment on the kick- at full extension, your body should form a straight line through the hip to the torso- you should not lean forward/down as you kick or you end up with a weird hip angle. Leaning back can make it a little easier to get the correct hip alignment.
The key is balance. You need to keep your center of mass directly above your planted foot in order to keep from falling, and if you are going to thrust a leg out in one direction, you need to adjust with your torso a little in the other direction.
If you want to do an experiment out to see why, try this: Stand with your heels and but up against a wall, so you are unable to lean back, and do a slow motion front thrust kick as high as you can. It is nearly impossible to do without falling over.
As a secondary point, you absolutely need your balance for a thrust kick, or else you will end up pushing yourself back as the other guy stands there and laughs (especially if he's bigger).
This may be a very simplistic way of looking at a side thrust kick, but I tend to view the leaning back of the torso as a by-product of the kick, not as something I intentionally try to do. My mental focus is on where my hips, balance, and momentum are. Where my torso leans is barely an afterthought. I recall when first learning the kick, I would consciously think about leaning back (no doubt to imitate what I saw experienced practitioners doing), and maybe my hips would still be in the right position, but at the expense of balance and/or momentum towards the target.