There are around 10 waza in Hakkoryu's nidan-ge that present an equivalent to nikkyo. Some are outside in, from outward appearances -- but the waza are designed to move from under and over while in, producing kuzushi before the lock is even set. This is why the pain is / can be a secondary motivator: the joints are locked all the way through the uke's frame, the uke is already off balance, so sayonara.
I was practising nidan waza last night in training for my next grading, a mixture of suware and tachi waza. One of my sensei's performed shuto jime (sword hand) on me and I just collapsed in a heap on the mat. There was some initial pain but then my knees gave way and before I knew it I was crumpled on the floor. At the end of the day pain shouldn't be the primary motivator because without taking kuzushi the wrist locks won't work. If someone strong really locked out on you and you didn't take balance first, you're never going to successfully apply nidan before your attacker punches you in the head. Taking posture is paramount and then the wrist lock will work, locking up the frame producing compliance.