There is something very strange with this video.
I've been studying it, as hard as it is.
The screaming starts at about 2:44:34(video time), and continues through all of the timestamp changes.
They knock him over at about 2:44:41.
At 2:44:42, the timestamp changes to 13:45:47, where he is apparently in the same position. The camera is zooming in at this time, while the screaming continues. It is apparently zooming out and at 13:45:52, it appears done. But if you'll notice, at that moment the timestamp changes to 13:45:59. It is at this time that it appears that you see him convulse once more. Without being able to see that missing 7 seconds of video it is kinda difficult to determine what was happening before the fact, but it looks like a true muscle contraction, and not manipulation by the guy holding him. It looks like the guy holding him has his knees in his back, so it would have to be pretty forceful. Due to this fact, I think he is alive at this point in the video. I do not think that the audio that you hear at this point is the true audio, though.
Ooh. Interesting. There is a 7-8 second gap between when the screaming starts and when the timestamp changes to 13:45:47.
Here's what I think happened. They actually started at 13:45:40 video time. For whatever reason, they cut out the 7 seconds of video from 13:45:52 to 13:45:59.
I'm not really familiar with video editing software and how it works/limitations/etc, but it's like they cut the 7 seconds of video out from 13:45:52 to 13:45:59, but kept the audio the same length, so it screwed the sync up at that point or something.
Hmm.
Anyway.. If you'll notice, the guy in the white mask is holding him down, and the guy with the black mask and knife(obviously) is doing the cutting. Between 13:46:20 and 13:46:28, there are a number of time discrepencies where the seconds move faster than normal. My guess is this was their attempt at shortening the video to get the audio to sync at least somewhat - at the most dramatic moments.
At 13:46:33, the timestamp changes to 13:47:46.
At 13:47:47, the man in the white mask holds up the head, and is holding the knife.