What is 'pressurizing' the medium that we are adding pressure to? Is it the muscles of the core- the dantien?
Chris, that is a good question. I don't think anyone has definitive scientific proof as to what that medium actually is.
People tend to think it might be the fascia as where it is present seems to overlap which what the Chinese traditional models point too (for example absence at the throat/anus/mouth). I have a hard time believing that various breath exercises would condition muscles or skin in the limbs, yet when you work on the pressure aspect, you feel this pressurization in the limbs as well as the torso.
While I do think that there is a mental process component that interprets these sensations from breath/pressurization conditioning, something in the body is likely to be changing as well. What that physiological component actually is, I'm not terribly concerned over as it doesn't effect my actual training. It certainly would be nice to know though.
I don't believe Mike Sigman refers to the joints themselves as being pressurized, though I believe Akuzawa sensei does refer to storing into them. Perhaps Rob or someone else can clarify what that actually means as I don't want to be tossing around buzzwords I don't understand.