Neat!
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"Polarity" is used loosely in the video. What the author did was to show how you can identify the foil side of a "non-polarized" capacitor manually using an oscilloscope. The technique is to hold the body with your fingers and reverse the leads. The lower amplitude connected to the negative lead is the foil side.
Early caps were marked foil with a band.
He then said the foil side of a cap should be connected in a tube/valve circuit to the lowest Z point. This makes sense.
Later, he went through a "design process" and showed the fruits of his success by making a gizmo that has two short shielded leads with alligator clips, and limits the output to +- a diode drop, and electrically reverses the leads using a version of the 4066, a Flip flop and a 555 timer powered by a 9 V battery and connects to the scope. A green LED lights at the side that should be marked negative when the amplitude is at a minimum.