It's a technical trick, first of all before you play the video it's 1:20 minutes long, but once you play it says it's only 55 seconds long. At 53, the demonic voice starts and it continues until 1:20 the original video's length.
Somehow it just looks like the video is over but if you try to pause it when it's fake over the voice stops so in reality it's just still going without you seeing it.