Re: does nikyo hurt?
I'd like to offer our local notion on jointlocks and their resulting throws:
Whichever of the controls you employ, the goal is to immobilize the movement of the arm and shoulder. When you have appropriately twisted the arm so that no slack for movement remains, the body is forced to follow the arm, when you drive it toward the ground.
This is how an effective nikka jo can be painless. You can push a rope or a chain, the saying goes- if you twist it just right, it locks up. Sanka jo is more ropey; nikka jo is the chain, in this example.