After finally watching the video linked, I can not offer my unbiased opinion.
I "see" four officers almost in control of the "suspect", when the fifth, (a coporal?) provided his "assistance" by shouting "Stop resisting, Mother F%^ker" and kicking the suspect 5 times, while the suspect was on the ground and almost under control. The assistance of those kicks was, IMHO, unneccessary.
As for the laughing, I do not know what "caused" the laughter. The tape skipped ahead, and did not include the moments before the laughter, so I do not know if the laughter was about the kicking, or in response to a "joke" by another officer. That portion of the tape, to me, was out of context.
The violence of the kicking portion is just wrong. Especially since the kicker was a corporal, and only outranked by the sargeant who initialized the stop. This man is going, or should I now say, "was" moving up in the ranks and possibly a supervisor or soon to be one. His example is worthy of his termination, or severe reprimand.
The laughing? I have to reserve judgement on that. Just missing too much info to know the exact reason. But, regardless, it was not "right"
JMHO