The aikikai shihan in charge for germany employs pain doing nikyo and emphasize it's use a lot.
this understanding of Asai senseis nikkyo-pratice ist not correct.
What he does is not about pain. The meaning is to learn to accept incoming forces, and to absorb them in your body. To pratice the wrists makes them soft, and you have to practice until you are able to take nages energy that brings you to the ground (if you are soft enough to prone position), where you can reverse ore open the spiral and escape, or do kaeshi waza, for exemple sankyo .
As an application nikkyo can be very fast, and before you feel any pain you are shocked, but you are on the ground already.