Last night we did nikkyo and I was so busy examining what I was feeling I forgot to tap out much to the consternation of Nage. Got a bit of a lecture for it.
Nikyo got me started with aikido, long ago. A friend practiced aikido, and I had no idea what it was, so I asked him to show me something. He demonstrated nikyo on me. I fell to the floor and started digging to get to the floor below. It was quite painful, but more than that it was amazing. Like magic. So, I had to learn it.
No pain no gain
By the way, Janet Rosen talks about "benign pain", which is a wonderful way of describing it. There most definitely is such a thing.