Just to disagree - If there were no pain, uke would not move, except to stop his wrist breaking. I think we get used to the pain and so think there is none. The danger is to kid yourself as to the reality. Sometimes, when I 'hear' or 'see' people teaching stuff, I just want to get up and ... eer ...show them how wrong they are. But alas, I am too polite.
I understand what you're saying, Rupert, and maybe a little of that is happening too.
All the same, I know what I feel, and nikyo delivered by someone who does it the way we are talking about does feel very different from nikyo done by someone who is not controlling uke's body structure.
We were practising this very technique yesterday evening (funnily enough), and I was experimenting with which part of use's body I was controlling through the wrist. If you aim your technique at the wrist, or try to control the front shoulder, this relies on uke feeling pain in their wrist for effect, and you have little control on their body apart from this (especially if they have insensitive wrists). On the other hand, if I try to link the wrist pin with uke's rear shoulder, they go down with much less effort on my part, and they tell me
they feel much less pain in their wrist.