There was a decent discussion of yonkyo in the Aikido section of e-budo in February of 2002. Below is a post I did on that thread, perhaps some one here will find a little value in it.
"The radial nerve compression has eluded me,……"
I just stumbled across this thread and have found it interesting and would like to share a few thoughts. I do apologize if I am repeating what someone else has posted; I haven't finished reading all of the posts but noticed the common statement above.
While I am certainly no expert I have found success with the nerve aspect over the last couple of years and MIGHT be able to lend some insight or at the least muddy the waters. LOL. At this point I would like to post a disclaimer: Painful nerve locks/pinches/etch don't work 100% of the time on 100% of the people. Using the radial nerve will work on the vast, vast majority of the people. There are a great deal more people that you will not be able to apply pain to the ulnar nerve (I am one) no matter what you do. Therefore I always look at the nerve pain as a bonus when it happens but I don't rely on it to accomplish a technique.
Several of the posts I read discussed footwork, posture, moving from the center, etc. which is all very important and well said so I won't repeat any of that. I noted that several people stated that the grip is like a sword grip, since I am not a swordsman I couldn't really say for certain one way or the other, however I would guess this to be not fully accurate. While the grip may LOOK similar, cosmetically, I doubt the manner of exerting pressure is similar, but I could be wrong. This will become clearer later in the post.
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. A 110lb women in spiked heels will apply a far higher amount of lbs/sq. in. to the floor with each step than a 250lb man in work boots. The vastly reduced surface area more than makes up for the weight difference. The 250lb man applies more overall force it is just spread out over a much larger area. This same physical relationship applies when working on nerves.
In general, most people tend to grab uke's forearm or wrist and attempt to squeeze it as hard as possible with all of their fingers and palm. This has two affects; first the lbs/sq. in. is minimal since it is spread out over a large surface area. This greatly reduces tori's ability to penetrate the soft tissue and affect the nerve. Secondly, since the pressure is spread out more or less evenly along the nerve (of the large affected area) there is no pinching action merely a mashing of a large section of nerve. Sharp nerve pain is caused when a tiny area of nerve is severely affected while the rest remains neutral. Sorry the theory is so boring but the rest won't make sense unless you understand the theory. You can run several self-tests by poking and prodding yourself using one finger then several fingers to feel the difference.
Now the trick becomes shaping and using our hand in such a manner that we affect the least amount of surface area with the greatest amount of pressure. Hold your palm out in front of your face, ignore your fingers and look for the longest distance between two points on your palm. The greatest distance is needed to create a lever action to apply the pressure. There are only two possibilities: diagonal from the base of your index finger to your heel of your hand below your pinkie, OR diagonal from the base of your pinkie to the heel of your hand at your thumb. Since the thumb is fleshy and soft it would absorb a great deal of the pressure we attempt to apply plus we would have a very difficult time creating the lever we need (don't worry if that doesn't make sense immediately). The point we will need to apply the nerve portion of yonkyo is the hard bone portion located directly under the crease where the index finger meets the palm. I guess you could say the palm portion of the index finger knuckle. Fortunately I have a childhood scar exactly at that point so it is easy for me to remember.
Anyway, this is the sole point we will be compressing the nerve with, the rest of the palm should have minimal contact with uke's wrist or forearm. Now we need to locate the second point on our lever so we can apply the sole point with force. The opposite point on the palm is the diagonal to the heel of the palm on the pinkie finger side. I think of a line connecting those two points, which creates a lever or bar. When we lift one end of a bar or stick the opposite end applies force in the opposite direction. We use a similar although not identical motion here.
Place your hand on uke's forearm in the sword like grip discussed, the point on your hand that will be pinching the nerve should be placed where you think the nerve is located. Tighten your pinkie finger round the forearm so it is directly across from the heel point of the palm discussed (the end of the stick). The other fingers and thumb should remain very relaxed and should NOT squeeze or apply ANY pressure. Actually, the index finger should be extended so the push point in your palm is hard and protruding slightly. Now keeping your wrist straight pull up with your pinkie and heel point and push down with the push point. This is a circular push/pull motion.
In this manner you should not be changing the shape of your own hand. Changing the shape of your hand merely adsorbs the force you wish to impart into uke's nerve. When done correctly the hand motion is accompanied with correct body posture and hip power. One should not feel tension in the forearm, biceps/triceps or shoulder, if you do then you are not using your body correctly. This will take a good deal of practice to become proficient, but since the basic concept of a circular push/pull is prevalent in many Aikido/jujitsu principles and techniques it is a helpful thing to learn intellectually. This sums up the basics with regards to use of the hand for yonkyo, there are some more helpful hints but they all require the understanding and proficiency of these basics first. This is a lot easier to explain in person than writing but hopefully some of it is helpful. Best of luck.
ps. I have never written part 2 of this post, which would be how best to expose uke's nerve when gripping. If you guys find this interesting and I get a chance later this week perhaps I will write that one.