This video really doesn't prove anything. I have also seen video where someone with a conceal carry permit, but otherwise untrained, failed at an active shooter drill. Take any untrained person and put them in a job that requires months/years of training and they will almost certainly fail at it. Cops spend ~6 months or more at the academy then are placed with a TO (training officer) for more months before they are allowed to patrol alone.
As for the original video it does not show the beginning and that's where the crux of the situation lies. Like what did the young guy do to get into a situation where he needed to be arrested? Did he resist or did the cop just think he was? I have no opinion on the overall situation as I did not see it, only this small portion. But I do question how the guy was supposed to comply with putting his arms behind his back or rolling onto his stomach if the cop was basically forcing him into the ground? He probably should have gone limp and not tense any muscles but it can be difficult when you are in fear. It really is hard to say what was happening and if the guy was resisting or not, only the guy, and maybe the cop, know for sure. And the slap/hit/whatever at the end was not really necessary as backup was there. The cop also seemed a little too wary of the bystanders yelling at them to get back or they were going to be arrested as well, like he was afraid they were going to attack him when his backup was there. Almost like he felt guilty for doing something wrong... Funny, since another bystander went right up to help and the cop barely said anything to him.
I doubt it is the case here since the guy was continuously yelling but if you are forced into a posture where you are experiencing positional asphyxia it is an almost automatic reflex to try to move so you can breathe. I have experienced it a few times without a large man on top of me and it was still quite scary.
Finally, who was the dude in the hi-vis jacket? I couldn't make out the patch on the arm.