I get the impression that the video is from the mainland, not Hong Kong. The device he's strapped in looks like a "tiger chair":
https://www.news.com.au/world/asia/tiger-chair-sick-torture-...
Edit: I was looking for a better version of this video on youtube. I couldn't find one, but I did find this English Agence France-Presse story that answers the question of why they'd release a video like this: https://www.youtube.com/watch?v=M8PgCUap1Vg
The benefit of releasing a video is the fear it provokes.
Not saying this is the case here, but obviously if you calculate outrage cannot grow substantially greater than it already is, might as well try provoking some fear.
Of course it might very well be false, this is just my idea as to why someone might release it if real.