Greg Henry (The Aiki Struggler) made the interesting comment on a recent post regarding kuzushi (unbalancing):
...I think that we need to have a better definition of ... kuzushi. Right now it seems like you're saying any type of unbalanced state is kuzushi. ... That seems a bit broad to me. Instead I suggest we use the term to mean a state from which you must take an unnatural movement to prevent falling down...
I've heard some instructors define kuzushi as "any time uke has to make an unintended step." The idea being, virtually everyone on the planet takes one step at the same speed, so if uke has to take one extra step, then tori gets what amounts to a "free move." It is as if uke is standing still like in the classic happo no kuzushi drill because he has to take that unintended step before he can go back to fighting against tori.
An instructor also told us a story in which he asked one of his Japanese instructors, "Show me an offbalance." The instructor replied, "You see that man walking there? Chance, chance, chance, chance...," implying that uke is offbalance on every step.
...That being said, you must do something, even if its getting out of the way to put someone [in] kuzushi unless they're doing some sort of insane off the top rope type of attack. All types of kuzushi occur pretty much by tori doing something, even though that something is often just relaxing.
I like to tie the facets of kuzushi together in my mind sorta like this: Uke is always offbalance when he is moving (unless tori is holding uke up), but tori is not always able to use that perpetual offbalance to his benefit, so tori must occasionally adjust uke's offbalance so that uke is off balance in some specific way at some specific time so that tori is able to use that offbalance.
That is a pretty minimal offbalance system that still provides sufficient kuzushi to apply anything in the Kodokan and Tomiki syllabi. The cool thing about that is, tori can apply this kuzushi model to uke on every step no matter how uke is moving no matter the size or strength difference. This is the true "happo no kuzushi" in the sense of "off balance in all directions."