Rolex describes the principle in its own technology: "In a constant state of unstable equilibrium, the half-moon oscillating weight is beholden to Earth's gravity… it rotates with the wearer's every gesture, and sways in one direction or the other." That motion is captured: "The energy generated by these oscillations is transferred to the mainspring, which is continually wound, through an innovative system comprised of reversing wheels and a gear train."
Winding in both directions. A rotor swings either way, but a mainspring winds one way, so the mechanism must convert both directions into one. The Seiko Museum describes its own solution, the Magic Lever: "regardless of the rotational direction of the rotor, the two pullets of the Magic Lever always generate a unidirectional movement and also make the transmission wheel always rotate in one direction".
An automatic is still a mechanical watch. Nothing is electrical; the wearer replaces the daily winding.
Sources
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