Re: Yonkyo nerve pinch
Seems kind of silly to me that most people think of yonkyo as just a nerve technique and spend so much time trying to learn how to do that aspect of the technique, all the while neglecting the much more important structural aspects. Any technique, and specifically any of the numbered wrist locks, can be done with or without pressure points. In my opinion, sticking fingers into pressure points does not make techniques more effective enough (if at all) to warrant the time spent learning to apply them or, more importantly, the extent to which they make the technique more painful to one's practice partner.
My personal philosophy is that I will not allow anyone to hurt me, on the mat or off, if I can prevent it. That's one reason I train in martial arts, after all. If someone wants to use pressure points on me he better have very solid technique, meaning kuzushi and center control all the way through (which very few people in aikido have, because it's usually not taught), or I will block it. And if someone has strong enough technique to control me well enough to get to the point where he can use a pressure point without me be able to stop it, then he could easily throw me without even using the pressure point. So I don't see much need for such training, and I think they often are a distraction from studying how to control someone structurally and maybe even a crutch for those with little or no understanding of how to do this.
From a structural perspective, yonkyo is actually quite a simple technique, certainly easier to do correctly than nikyo or ever sankyo, and perhaps even easier than ikkyo.