The guy was making threatening movements (twitching from taser) so the officers out of fear for their own safety (3 against 1) used tactics taught to control a subject (stomp on heads,legs,neck,groin)
"In the beginning of a change, the Patriot is a scarce man, brave, hated and scorned. When his cause succeeds, however, the timid join him, for then it costs nothing to be a Patriot."