Here is a link to the longest version of the video I could find:
https://www.youtube.com/watch?v=DcCN5C7UBx0
At 0:14 her knees bend, facing away from the cop, and she goes down. It is hard to tell with the blurriness and shakiness where the cops hands are at this point.
At 0:17 it looks like she is pushing something red in the cop’s face.
At 0:19 she is trying to pull her knees up under him and is grabbing at his face and neck.
At 0:21, after trying repeatedly to restrain her while she resists, the cop throws the first blow. At 0:22 he swings his right elbow and forearm forward which might have also been a blow.
At 0:29 she pulls her hands away to cover her face with her arms.
It appears he lands a total of 9 direct hits (at least one is obscured by the car, but possibly two are) up until his last blow at 0:30. (About one blow per second for 9 seconds.) As soon as she stopped resisting, he quit throwing blows.
At 0:31, as soon as the blows stop, she looks up and reaches toward his groin with her right hand.
Looks to me that she was trying to get away from the officer, he had to chase her down on a busy street / freeway, she resisted arrest, either threw herself down or fell while trying to resist, possibly kneed the officer in the groin, possibly scratched and hit him, and clearly continued to resist. It took the help of another man to restrain her as she continued to resist even after the officer stopped throwing punches.
It was only after multiple blows that she switched to a defensive posture covering her face. When the punches stop, she immediately starts grabbing at the officer.
In my opinion, it is impossible to tell from the video alone if this was excessive force. It warrants an investigation, but both the cop’s and the woman’s histories should be taken into account.
If there was evidence that immediately before this the cop had sexually assaulted the woman in his police car, then this incident would obviously incriminate the cop. But if immediately before this the woman was walking through traffic and nearly causing crashes upon which the officer tried to apprehend her, it would show the woman needed to be restrained even if the only way to do it was to throw “brutal” punches.
As soon as she stopped resisting, he quit throwing blows.
That’s good enough for me.