The most common problem I have seen in people attempting to apply painful pressure to a nerve is that they mash the nerve instead of pinching the nerve.
I agree whole heartedly. I think that those of you who have a large bruise on your forearm after someone has applied yonkyo may have had someone who did not practice the subtleties of this technique.
When I teach it, I emphasize not only the position of the compressing knuckle, but the relaxation of the grip. I think of it as a loose ring around the entire wrist that only compresses when torqued. The pinky and palm make up the bottom of the ring and the index finger and thumb the top. Similar to one of the tension devices on a tent rope.
In fact, I generally demonstrate it by having the uke hold their arm up in the appropriate position and then make a large ring of my two hands with index fingers touching and thumbs touching (picture ittsy bittsy spider) and the uke's arm inside the ring. I don't touch the arm when the ring is perpendicular to the long axis, but as I move into a more vertical position, the large, lax ring becomes tight and gives a good yonkyo.
This is not they way I apply yonkyo, but it demonstrates to the students the might doesn't make right when it comes to yonkyo.