Yonkyo nerve pinch
Like Jun, I go for the kuzushi as the primary effect of yonkyo. I have a guy in the dojo who has done karate for years, and yonkyo is not very effective on him. Nikkyo and sankyo work fine, however.
?The radial nerve compression has eluded me,???
There has been a lot of discussion as to how yonkyo works, and I don't buy into this one. If you bang the same nerve at the elbow, the sensation is quite different. Sometimes yonkyo has a similar flavor, but sometimes it hurts, but doesn't.
I'd say the parallel with a sword strike is more related to the mechanics of lowering the arms than in the actual grip, though I am not a swordsman either.