I don't believe the pain is "incidental" in yonkyo. I do believe the pain is transitional. If uke moves how and where nage wants it doesn't hurt.
A trick I show my class is sitting crosslegged. Put the first finger pad on the inside of the opposite ankle and project energy through your finger. It lets people feel yonkyo on themselves.
Having it done repeatedly on me I have developed a bit of an immunity but when someone projects their energy in the fashion of a sword strike ( or bolt lock) I drop like a stone.