"Painful grip" seems like a pretty well centered shot. Maybe you were referring to off center because the subject is upside down, I could see that. I think the full frame seeming pretty centered might be why the score was not higher.
The shot itself is amazing in its timing. I really like the motion that you captured with the slow blur/glow around the movement of the legs. The composition is great. Even the composition really isn't effected by the lady in the background watching because it is such a strong subject. Also good use of diffusion to soften the edges and allow to focus to remain on the subject. This really is an awesome action shot.
I love it. I absolutely with out a doubt love it. A longer shutter speed and firing the flash at the second curtain would have made this one unreal. Great capture.