I had a very interesting conversation with a Hakko ryu practitioner earlier this year. The gentleman showed me that it is possible to lock a joint without causing pain or causing uke to feel much at all. He locked my wrist softly and then rather than continuing to apply pressure, he locked my elbow softly. Again without continuing to apply pressure he took control of my shoulder and I had to go prone on the mat to deal with it.
The structural alignment used in Hakkoryu is different from that used for nikkyo / nikkajo to which I've been exposed in aikido. Hakkoryu embraces inducement of pain, but puts a premium on applying wrist locks in manners that control the whole body from the onset.