Re: If it has to be felt, what does a feeling look like?
I think it would be necessary to place "sensors" inside. But I even if this would be possible, I don't think this would lead somewhere.
When I attended a seminar of Ikeda Hiroshi he enjoyed to not move one inch but to change "his inside" so uke lost his balance.
I have some uke who I can lead to a certain point and get to a position where I can throw them or not throw them without moving. I change inside. But also I don't think I'm "moving" inside. A sensor would stay in the same place. But maybe it would change it's "direction".
Also: Endo sensei teaches to lead one's feeling downward. This can not be shown. And this is not alone about the alignement of the body but also about directing the intent down, to the earth.
Maybe it would be interesting, to mark the vertebrae of the spine. Or some point within the shoulders, the hips. To see how all this opens/closes, twists ... But I don't think it would help.