you can do it several ways. but the common thing for different nikkyo styles is the transition of the grabbing hand going into nikkyo.
This clip is one of my favorites for nikkyo ura with miyamoto sensei.
http://www.youtube.com/watch?v=H2qiL...feature=relmfu
and another one with endo sensei.
http://www.youtube.com/watch?v=kZ3AKzd6FPU