Re: Kuzushi for kaiten nage
I definately tend to cut at the elbow rather than the wrist. The wrist is at the end of the lever but without control of the rest of the arm the elbow can be used to compensate for movement at the wrist.
There's a couple of ways you can do this. There's the long version where you can tenkan and underhook the arm to turn them around as you then step under the arm for sankyo or kaiten nage.
For the more direct version (which it sounds like is what you're talking about) the breakthrough for me came when I was told to cut not with tegatana or knife edge as with say irimi nage, but to place the back of your hand on the elbow. This results in more of a cut outward on the arm than downward which facilitates kaiten nage much better.