Apropos of "rooting." I have said before that what Mike and Dan describe seems to me to be basically kokyu tanden ho. And lest I be accused of cherry-picking -- this recently was given in a post recounting Second Dosshu's lecture on Ki development, in which Kisshomaru explicitly discusses "rooting to the earth." Originally from aikidoonline. I think it gets pretty plainly at the nature of the means intended to develop a "rooted" center with aikido practice, as distinct from doing "rooting" to develop centering as a predicate for aikido practice.
This part precedes that discussion but is also of note:
Shioda's preemptive movements in the shoulder push video are very illustrative of the approach Kisshomaru Doshu describes, to my mind. "Preemptive" is precisely the right word, too, for those who are translation junkies. Pre + emptore = "to buy before."
Shioda buys into the attack completely -- before it is completely delivered.
Putting aside the fact that Kisshomaru isn't a good example either, (particualy since I think he only got this stuff on a very very very basic level)
the whole concept of go no sen, sen no sen etc etc, is simply application of the baseline skill sets.
You practice recieving/grounding to develop the skill to some degree. There's other ways to develop and refine as well.
But whether you chose to pre-empt (sen no sen, you kenjutsu freaks correct me if im wrong) or simply react (go no sen), is a difference in application.
Digging into the whole moving on "Setsuna" is interesting, but only if we have the *dum dum duuuuuuuum-* baseline skill sets at play