What an excellent thread so far!! George's post is enough for a nice magazine article by itself!
Not much I could add, other than to share a story. I've practice kotegaeshi with tae kwon do people, and one I was working with had a lot of tension in his arms and wrist. I always try to finish a technique, so the next time I applied atemi to the face which worked (once). He was pretty firmly rooted the next couple times, so I finally feinted a front snap kick to the groin - he screamed, bent forward very fast, totally relaxing his wrist and then went down with kotegaeshi real nice. Not elegant, I admit, but it worked.
The inward squeeze of the finger in a fist described works real well, too.