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Explained: https://www.youtube.com/watch?v=CqOoY-o-j-Q

He does some processing on the area of the footage with the crt's to make them look nicer on video. You can't actually see the screen, it's just a square the same size as the monitor that seems to be making that region darker.

My guess is he was using a filter on a portion of the screen to reduce glare or something like that in post production.