I don't find this as compelling as others seem to, but I seem to be in the minority. So here is the video you've all seen already.
The cop seems to have a bad case of Respect My Authoritah, but I don't find it as clean an indictment of him as I might like because the driver turns his body three quarters away from him -- which can be seen, arguably, as a provocative action. Who knows, after all, what he may be doing when his body is turned and hand not in full view.
That's not a defense of the cop, who seems to have acted violently due 90% to his belief he wasn't being afforded proper respect or deference. I note it more as an argument that can be made at trial.
Though trial or not, he's gone from the force, I think it's safe to say.