The video is clear:
The young white man was surrounded, grabbed around the neck and thrown to the ground, whilst on the ground, another of the gang punched him the back of the head. He was under attack. The female police officer then grabbed the victim around the neck, amidst the attack, and he lashed out. CLEARLY, he didn't even see who he was hitting. The police officer then called the victim "a dick" before arresting him, and none of the men that had assaulted him.