In aikido (and to some degree in judo) when tori throws, he pushes/pulls uke to extend uke's center of balance beyond his base of support.
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So long as uke has a foot on the ground, he has some degree of strength and control over his fate. Tori must push uke sufficiently far through some arc over a foot that he is unbalanced enough to give up his footing and fall.
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If uke is unfamiliar with being inside that arc (post-kuzushi but not yet falling), he might be so uncomfortable that he gives up his footing at the very beginning of the arc, before he has been pushed very far - basically jumping into the throw as soon as he is pushed. But if uke becomes more familiar with that arc and more proficient and confident with his falling (and surviving) skills then he will be able to wait longer - to get farther into that arc before he has to give up control with his foot and fall.
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Extending uke's arc makes uke better because it gives uke more more control for longer. He may be able to use that control to reverse the technique. It also makes tori better because it forces tori to have sufficient stroke to push uke far enough to insure that he falls (or in other words - to control him all the way into the ground).
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What if uke were able to extend that arc all the way into the ground? What if tori pushing uke into the arc was just the beginning of a French curve that ends on the ground. What if uke were able to allow tori to place uke's upper arm or shoulder all the way on the ground before he had to give up control and fall?
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Aikido is not a discontinuous aiki-then-ukemi process. There is a continuity in aikido that extends seamlessly all the way through your ukemi.
