Although I practice a different style, nikkyo (in yoshinkan its nikajo) is a very difficult technique to perform correctly despite how easy it looks. In Yoshinkan we try to keep our hands in kamae (arm stretched out infront of your chest). One of the key aspects of the technique is to NOT twist ukes wrist. I have been taught to cut down like a sword. Use your lats to cut down and shuffle in slightly. Dont bend your wrists. The idea is to not twist, but to unbalance uke through his/her shoulder by cutting down. Uke should feel no pain, he/she should become unbalanced from this. Dont think about the wrist, focus your power on the shoulder. (its kind of difficult to explain it in writing)
I hope this explanation helps. I havent gotten it to work pain free yet since its a suprisingly difficult technique to grasp.