This year we are discussing the Book of Martial Power (BOMP)
Most of the power that we are able to put to use in martial arts applications comes from our interaction with gravity and with the mass of the Earth. The placement of the point(s) where we interface directly with the Earth (our feet) is obviously important. The placement of our center of mass relative to our feet and the line of gravity is also important. Obviously, the way that we place our hands on the opponent is important.
But an often overlooked, vital linkage in the power chain is the alignment of the spine. It is through the structure and musculature of the spine that power is transmitted back and forth between our arms and our legs, or ultimately, between the Earth and the opponent. If we can manage to place our feet and our hands right, and to align our spine correctly, we can serve as a very efficient conduit between the opponent and the Earth.
What is proper spinal alignment? I was brought up in the old rigidly-upright posture school of thought. The idea that if you have to lean to do it, then you don't do it. Or, another way to think about it is, if you can't do it with a perfectly upright posture, then you can't do it.
But in the past few years I have come to think that there might be some acceptable deviation from that perfectly vertical posture. These days, I think that we generally want to operate in and around a generally upright, comfortable, natural, neutral posture, but that we can (and must) deviate from that to some degree in our movement.
But how much is it permissible to deviate from vertical? Generally I think that your torso can bend forward at the hips so long as the torso remains one stable structure - the torso does not bow and flex like a noodle. And I think that a good, common-sense limitation to this forward bend is however far you can lean and keep your torso between vertical and the line of the back leg. So, you might, if you have to, lean forward until your torso and your back leg make a straight line...
But it would be better if you moved your feet instead of leaning - generally, if you have to lean then it is an indicator that you got the preparatory footwork wrong.
So, the lean is a backup plan for when your feet don't end up just exactly right. But you have to get the feet correct enough that you can keep the lean to within functional limits.
It's sort of like those infuriating delta-epsilon proofs in Calculus I - where you just have to get one variable sufficiently close that the other variable stays within the bounds that you want. Crikey! I never thought that I'd ever see a use for those awful things!