As I see it the continuous effort is to put uke on the edges of the feet as you move, making them unstable though not out of balance. It is hard for them to move from this and they don;'t really get a sense of being out of balance. The out of balance comes with the completing movement...one they can't resist and have no time to move to get back in balance.
Yes, in my experience it's either getting them on their toes or getting them subtly relying on me for their balance. That tiny shift of kuzushi can be almost imperceptible to uke right up to the point when you "let" them fall or lead them in to the fall. It's kind of like dropping them in to that third leg they wish they had right underneath them in that hole they didn't realize was there...