It's on the longer clip:
Here is the lead up to it
The cop knocks her down at about the 3:10 mark, and he hauls her up by her hair at the 3:15 mark. He then forces her head down and gut-punches her 3 times. It does not appear that she said or did anything that would warrant a beat down like that, and the bystanders all look and sound pretty shocked.
Let me tell you about a situation involving a soldier in basic training:
In a breaching training exercise, four soldiers in my training unit went through a mock-up of a house in a mock-up of a town. The rifles were... I don't really remember at the time. Paint rounds or airsoft or something on those lines. There were both civilian panels and non-civilian targets. They'd pop up and you would have a fraction of a second to shoot or not.
There were a few doors, some of them locked with a special kind of hasp made for a heavy plastic sheer pin, about the same strength as a modern interior door. Those could only be opened using a portable ram.
Four soldiers would form a breaching team. The goal was to get 100% of unfriendly targets and few-to-no civilian targets. Every group had a round or two in a civilian target by the end of it. In addition, one of the soldiers in charge of manning the ram breached a room and then walked in with his rifle facing the wrong way, not even noticing until after he put the thing down. That particular soldier didn't hear the end of that incident; the soldier in charge of the range it happened on got assigned to my unit, and even three years later talked about that 'idiot soldier who walked in with his rifle backwards'. But I knew the guy it happened to; he was a decent soldier and just got mixed up; and with an m-4, the barrel is as long and only slightly more narrow as the butt, with handle-sized bit going each way. I can certainly see how easy it would be to make the mistake.
Adrenaline reshapes our perceptions, which are already fragile. It's entirely unreasonable to demand that anyone, particularly a cop reacting on behalf of a person calling for his help, be entirely aware of every detail involved. That includes being lenient on his possible failure in identifying that the person he tackled was a teenager.