I think this is because there are certain positions where if you put that body in those positions he is unable to adapt or regain his balanced state. An example is the upturned palm during the ikkyo pin. An ikkyo pin can actually be done using just a thumb on the elbow as long as the palm is facing upwards. With minimal contact, uke won't be able to move. But if the case is that uke's palm is turned down during the pin, he can easily regain his balanced state no matter what weight you put into the arm.
This is not mechanical advantage in the sense that the resistance, fulcrum and lever are in the same locations for both cases, the only difference is uke's palm if it is up or down during the pin.
What do you think is happening in this situation that makes it difficult for Uke to get up from the pin? What makes turning the palm up or down have this effect?
I'm asking because I would like to know more about what you think the phenomenon causing this result is.
From what you just described, I would say you are describing something different then what I was getting from other people when I made an attempt at creating definition "D". To me, Definition "D" has a feeling that nage has trained his way to correctly make adjustments to the situation. But what you are describing, at least from what I read is related to something going on inside of Uke. This is why I said it seemed to me like "proper technique" and mechanical advantage. Maybe mechanical advantage wasn't far reaching enough, maybe I should have added "and/or inherent mechanical weaknesses in the body".
If I were going to make a quick definition of proper technique, maybe something like:
Proper technique: Taking advantage of inherent mechanical weaknesses in the body by means of a superior mechanical advantage.
This definition would allow for things that exploited either nage's superior mechanical advantage, or uke's inherent weaknesses (maybe what's going on with uke's palm in your description?).