So this a practice that aims to refine the body of uke? To get the "dust out of the joints" as o sensei called it?
So when students of Asai sensei told me, that "nikyo has to hurt" this was about uke's way of receiving nikyo and not about tori applying nikyo?
Yes. I think it primarily is a practice for uke. If you feel pain, you just have to practice more.
One aspect is to get the dust out of the joints. Another aspect concerns the need to be able to absorb any affecting forces. Thats why we practice nikyo, sankyo, yonkyo a lot.
But I never tell people that nikyo "has to hurt". I tell them if it hurts, they have to practice more, so that they someday come to enjoy the feeling as "massage".