I think officers must be coached to yell "Stop Resisting" while beating people (who aren't resisting). I've noted it a few of these incidents.
https://www.facebook.com/CopBlock/posts/10153729801870507
https://photographyisnotacrime.com/2015/06/california-cops-c...
http://www.nydailynews.com/news/crime/police-dash-cam-video-...