Traditional animation involves drawing the outline of the characters in black ink on transparent film. The plastic sheets have holes near one edge to allow them to be aligned on top of an earlier frame so that the outlines can be drawn near the coresponding line on the earlier frame. At that point the areas inside the outlines are still completely transparent.
The color fills were added in a subsequent process. The inks used for the fills are formulated to produce an even shade so that there's no unintended motion effect within the individual filled area. Because of this only cartoons with extraordinarily high labor budgets attempted the effects like shadows on the lower side of the character's limbs.
Occasionally someone thought it was an "artistic" effort to fill the character's with an uneven media resembling color pencils. Since the pattern in the shading was different from frame to frame that process produces a vibrating effect withing the fills.
Ah ha, I see. In one commentary, Matt Groening was talking about when they first started to shade the characters, and it was supposed to be groundbreaking. Until your post here, I wondered why it was so groundbreaking, and now I see why. Your post also answers why I don't see animations with wavy lines in the figures themselves, something that was stumping me. Thanks, Robert