... standing on a jo, moving so the full pressure hit every point on his feet, and rolling the jo back and forth with pressure, as if grinding out the muscles of the feet.
Interesting. My first teacher used to do this very often.
Im was nearly not able to do it correct or completely, because there where some places on the sole of my feet where I just could not bear the deep pain. It felt as if my feet would break apart, or as if the pain was going deep through my whole body.
I am now doing kind of qi gong since some weeks. And standing "correct", making contact to the ground, aligning the body and so on, causes a little ache here, a joint that creaks there and so on. The body "works", just like timber.
And regarding my feet it is the exactly same places I "feel" during specific excercises.
There is a whole lot going on down there in my feet. And whatever it is, it is clearly related to what is going on in the rest of my body. ;-)