I agree, the video shown appears to show this effect, so I'd be interested in seeing this particular segment in the original PBS footage to confirm it wasn't manipulated. If it's the same in the original, I'd have to lean toward green screen. Without that, I'm very skeptical.
The only other rational explanation I can think of is video compression techniques, though to be honest I've never seen compression cause this kind of artifact in this kind of scene. Video compression likes to re-use slow and non-changing portions of surrounding frames to reduce filesize, which COULD potentially explain the missing foot, as well as the seemingly still audience.
Yeah, people don't realize how sophisticated video compression is these days, and how high of a level it operates on the image, so you can get weird things. But this artifact doesn't make sense along those lines. Looking at the high-quality video on YouTube it doesn't seem that high compression was ever applied, so such a glaring effect wouldn't be present. Maybe there was some data loss in the stream and it patched it up, but that seems unlikely.
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