The Supreme Court has stated that the police can order people on a traffic stop into or out of a vehicle for any reason. The officer tells her to get into the car completely so that he can close the door. When she refuses, he tells her to get out, but then she refuses to do so. At that point, the police are allowed to use the force necessary to make her comply.
Now, at that point he appears to want to take her into custody or at least detain her and she resists, so he is allowed to continue to use force to detain her. Even after being on the ground, she continues to try to get up when he is giving her commands. I don't have an issue with the force he used.
Here's where I have a huge issue... His comments are completely unacceptable! There is no room for that kind of attitude or belief in police work.
The other issue I have is why she was being arrested. While the police can use force to detain someone, just someone resisting being detained is generally not sufficient for an arrest. Resisting arrest is just that. It means you resisted being arrested, not being detained. There are plenty of situations where the police use force to detain someone and in the end the person is let go without any charges.
In this case, from what I see in the video, she should have been detained, issued the summons, and then sent on her way. In Virginia, if she had refused to sign the summons at that time, she would be taken in front of a Magistrate to answer why (signing the summons is a promise to go to court or pre-pay a fine, if allowed).