If you don't mind, these are JMO.
I do like the shot and concept of this, I think it may be a little on the dark side for me, but that would be a personal thing I guess.
As for capturing the movement, I think you have a good start with it. Tai Chi is about fluid movement if I remember my martial arts right. Been waaayyyy too long.
Think of it as a form of ballet or meditation in motion. Some movements are explosive yet suttle. As with my steer roping shots, I never know where the action will be, but I know the movements that will take place. I believe photographing any martial art form would be the same. If she is in motion, try different view points to shoot from.
If the legs or arms seem out of sorts, maybe a different lens is needed.
Capturing any sport is finding the right viewpoint I feel.
If I'm off base here, I hope someone let's me know. (ducking before Christian throws a smarta$$ remark) [img]/ubbthreads/images/graemlins/smirk.gif[/img]
The right view gives the desired feel to it.