Automatic winding springs from a simple principle: an oscillating mass, the rotor, moves with your wrist and transfers energy to the mainspring. The real technical distinction, though, lies in how that transfer happens — and here solutions diverge considerably.
The classical 360-degree rotor has been the dominant solution since 1931, when Rolex introduced the Perpetual system with bidirectional winding. Before then, John Harwood had attempted a sector rotor that oscillated about 180 degrees, stopped by shock-absorbing springs called bumpers: it worked, but with lower efficiency. Bidirectional winding, which recovers energy in both rotational directions, is now the standard in quality movements. Unidirectional winding, which transmits in only one direction, allows in certain cases faster spring tensioning but operates on half the available motion.
The microrotor is a chapter apart. Universal Genève developed it in the Fifties: instead of a semicircular rotor covering the entire plate, a small-diameter disc of high density is integrated into the movement's thickness itself. Tungsten is the preferred material — density approximately 2.5 times greater than steel — because it allows the necessary mass to be concentrated in minimal space. The result is visibility of the internal decoration and reduced thickness, two objectives a conventional rotor compromises. Roger Dubuis has also used two microrotors in parallel on multiple complications, a rare solution but coherent when the central space is already occupied.
IWC pursued a different path with the Pellaton system, patented in the mid-twentieth century and still in production in the caliber 82110: cams and levers convert the rotor's rotation into stepped transmission with reduced wear and high efficiency, the basis of the Portugieser's seven-day power reserve. Ulysse Nardin responded with the Grinder, a flexible-frame system that multiplies even the smallest wrist movements; introduced in 2017 with the InnoVision 2, it achieves double the charging speed compared to traditional systems. On the opposite front, Urwerk developed turbine systems — in the UR-150 a double counter-rotating turbine regulates the skeletonized rotor's speed by absorbing shocks, converting what is normally a mechanical problem into a control mechanism.
At the bench I've seen that most insufficient-charging problems don't depend on the system chosen but on the condition of the unidirectional brake — called a sprag or ratchet wheel depending on the generation. When that slipping wheel begins to wear, the rotor turns but energy doesn't reach the barrel. It's a silent failure: the watch slows down without obvious signs.