@theluthier You just said at around 3:32 to test the upper eyelid's movement with "turning off" the "Proportional Edting" but did in fact the opposite. The explanations after this confused me at first a little bit, but then I got it what you intended: You rotate the three selected vertices of Piero's left eyelid with "Proportional Editing" turned on ("Sphere" falloff) around the cursor set to the center of Piero's left eyeball. It's only important to switch off the "Mirror Modifier" on that eyeball so that "Cursor to selected" places your cursor in the center of that sphere and not in the middle between the two eyeballs.
I had to listen back a few times and I'm pretty sure I said "turn ON proportional editing". But it's definitely not clearly enunciated. Sorry about that confusion!
The cursor to selected operator shouldn't account for the mirror modifier's mesh elements. Only if the eyeball's mirror modifier is applied will that operator account for mesh elements on both sides of symmetry.