: : Gee, could I just put the handcuffed person into the back of the car?

You don't just put someone in the cruiser upon cuffing. First you must make sure they have nothing on their person. This is done for safety(to make sure they have nothing to free themselves, injure themselves or injure the officers)reasons aas well as prosecutorial reasons (make sure they don't dump evidence in the car seat) and Liability reasons (again injuring themselves or making up a story that something in the car injured them when they had it on their person all along. The vehicle should as well be searched to make sure of there being nothing in it for the very same reasons.
I have only seem the video three times (not near enough to truely speculate) but it does appear to me that the officer are trying to perform a search of the detainee prior to placing him in the cruiser. It does appear the the detainee has movement about his lower body prior to the strike to the upper body. If you look closely it appears that his weight shifts from his lower body to his chest and shoulders, indicative of someone trying to kick or move their lower body from that position. We shall find out soon enough as I believe the officer was indicted late yesterday on one of the counts.