Well, I'm thinking of the observations by Ellis Amdur about how that conditioning can come about after years of ukemi. How the very mindful practice helps develop that sensitivity and conditioning. So we have people with a zillion hours on the mat who can do amazing things. Maybe it was those zillion (appropriately spent) hours on the mat that *allow* them to do those amazing things today. And feel like "hey, it's nothing I developed, I just could do it one day... How come you can't feel it?"
Just thinkin' out loud...
When I looked around the web to figure out wtf the "silk reeling" thing was talked about in another thread here, I saw, a couple of times, a theme that i think is classically Internal Chinese about how chi is soaked into the body and cooked over time.
That had me thinking about the whole "ukemi softens the joints to allow for the expression of takemusu aiki" theme of Hidden in Plain Sight.