for the flashing on/off, they've got the camera on a tripod, and they're just swapping the footage with him in it with a still of the scene without him in it, back and forth so he's there and then not, there and then not.
the second effect is just them pausing the footage with him in in, and it dissolving (probably not the right terminology, but meh) into a frame without him in it, so nothing changes, everything else replaces itself, but he's not in the second shot.
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