Yonkyo, (Slightly) Demystified

I recently started learning yonkyo and was utterly failing to understand what was supposed to be happening. So I went on the tubes, hoping to find a decent explanation of the actual mechanics of the technique. Being utterly underwhelmed by what I found online, and having it recently explained to me in detail by someone who knows what they're doing, I'm putting it down on paper for the edification of future generations.

Here's a picture of the bones and major nerves of the arm; note in particular the presence of the "superficial radial nerve". This nerve runs along the radius for some distance, very close to the surface of the skin. The essence of yonkyo is to cause a lot of discomfort by compressing this nerve against the bone.

The mechanics of how this is done are pretty straight-forward; HT to my instructors for their explanation. So:

• Imagine that uke's arm is like a bokken, where the hand is the hilt and the inside of the arm is the back of the blade.
• When applying yonkyo to uke's right arm (like the diagram above), nage should first gain control of the limb by gripping uke's wrist with eir right hand, as if nage were grabbing a bokken.
• nage should then place eir left hand in front of eir right, again more or less as if ey were grabbing a bokken.

nage should end up with the proximal phalanx and/or middle knuckle of the left index finger conveniently spanning the superficial radial nerve. To apply yonkyo just sequeeze so that the phalanx/knuckle compresses the nerve against the bone. Doing this is highly unconfortable for uke, which is the essence of the technique.

Note that the above completely ignores how do you get into that position and what you do once you've got it; there are plenty of videos out there which show that type of gross detail.

I'd also like to note that I'm typically very skeptical of nerve/pressure point techniques in general, since they're often difficult to execute well consistently. Yonkyo seems to be an exception, since the nerve is easy to find and engage once you know its there.