On one fellow to whom I applied a hard yonkyo there developed a bruise that covered most of the inside of his forearm. He was very susceptible to the yonkyo pressure point. On another fellow, however, I had to extend into yonkyo until his arm fractured before he felt any pain (he didn't realize it was actually broken until several hours later). So, I wouldn't rely solely on the pain of yonkyo to control someone. It's better, I think, to use yonkyo to take uke's center than to crush his forearm.