I don't have time to discuss it in deep right know, will come back later. Only quick overview - if uke is not passive, he can regain his balance even after being totally unbalanced. That happens because nage is always making errors.
With traditional aikido practice we may have impression that every technique is successful -- Uke always helps Nage more or less. But when Uke become very active, suddenly all delusions of grandeur go away and we are faced with painful reality -- our techniques always sucks.
I think that most people put the cart before the horse - any technique is not going to be useful (or even possible) unless you can stand on the stable change point. I was working out with Sam Chin
when he said "You don't know where your center is - don't Aikido people talk a lot about the center?". I said "Yes, they talk
a lot about the center.".
Sam, FWIW, had no problem taking anybody's balance - while maintaining his own just fine.
Anyway, the basic characteristic of the change point is that it always changes - and training to address that needs to happen before you can even think about applying a technique.