The basic set of definitions in Nelsons (admittedly, not a specialized dictionary for Chinese medical usages) has the following:
Nelson 3799: koshi hips, loins, waist, pelvic region; small of the back; haunch; lower-panel wainscoating (lower wall only)
Afterwards come the compounds. So, a couple of questions:
First, do you have a citation in which it is quite clear that Tohei (or any other Japanese martial artist) is using koshi in a way that refers specifically to "small of the back" and not the entire region, which includes "hara?"
Second, while all these points or regions play a role in direction and alignment, how could a ground force "originate" or be "centered" someplace other than the ground? Wouldn't that make it some other kind of force if it were?
Good talking point.
Story of a session with a venerated, very talented and famous Japanese teacher:
Students seeing him turn from the waist and twist during all movements and he tells them "Move from Koshi."
Note* The teacher says koshi
in Japanese but knows the english word, hips.
Students ask "Sensei you say move from the hips and yet when you move, you turn from the waist, leaving the hips more or less square. Which is it; waist or hips?"
Teacher does the Kata again and looks down and looks up at his students confused. He slides his hands up and down from hip line to sternum and looks at his students and with a big smile says.... "Koshi!"
Years go by
Two students under the same teacher go different ways. They meet. They teach completely different body mechanics. Both watch each other and say....
"You didn't get it."
Good luck with that.
What is key is that what most people would call their hips, is not what you really need to move down there for power, or what and why it is important to move their waist in the first place.
I read all of it and shrugged. Everyone is going to go back to what they were doing when they walked in the door.