The problem I have is with taking sankyo on the hand with one hand, then using the palm of your other hand to push the fingers back. It simply adds more pain and people tend to rely on that rather than improving their overall technique. Just my own take on things fwiw.
I was lucky in having an instructor some yrs ago who would have us all gather round and we'd study the anatomy/kinesiology of the various locks as he very slowly applied them on somebody, starting with the contact point and progressing up and to the center. Then we'd practice just that part, isolated from attack or completion of technique.