this occurred in the hospital waiting room
Nonetheless, police officers already have methods of dealing with this sort of behavior. It may well be that some variation of "put her on the floor and sit on her" was called for here, but I can't imagine body slamming someone who's handcuffed is called for.
I'm not a cop and never have been, though I know a few and have worked with them in situations similar to this. I'm also familiar with the training orderlies and psychiatric aides receive for handling psychiatric patients. I can't say the two receive identical training but I can say they approach things in general very much the same.
A psychiatric patient who was already restrained but still lashing out would, if the orderly/aide was alone, be put on the floor and held there or held her in place from behind in a standing position, until backup could pitch in. Then she'd be relocated to full restraints on a bed in seclusion (at worst) or place in seclusion in body restraints (at best)
if she wasn't able to control herself after having been held down or held in place. At no point would she be body-slammed as in the video.
Again, can't say the police are trained to do precisely that, but I'd bet their procedure is more or less the same. Police officers are allowed a bit more aggression in their handling of these things, but in all cases the standard is "do the least harm".