Here's my take on the video: The rotor creates thrust downward. The cardboard attached to the skids acts as a "deflector" changing the thrust direction to a radial one. Thus, left cancels right, forward cancels back, etc. I also seem to recall the Harrier and other VTOL aircraft were prone to "recirculation" near the ground. This occurred when the thrust was taken up into the intake in what amounts to a nested loop. With the thrust being directed so close to the blades I'm compelled to think that what little pressure difference is being generated between the top and bottom of the rotor is being undermined by this very same recirculation effect.

BTW, I found this video much more interesting than the airplane on the treadmill which seemed to generate endless discussion. I admire this authors knack for experimentation. (and I also enjoyed that link Paper Cub!)

Crate