The micro-rotor is an automatic winding rotor with a reduced diameter, integrated into the movement's plane rather than overlaid on it. This distinction is key to everything: the conventional rotor covers most of the plate and adds thickness; the micro-rotor lives within the movement, freeing up both the third dimension and the view of the mechanism from the caseback.
The technical logic is simple to explain, less so to execute. To generate sufficient torque with such a small mass, you need a high-density material: platinum, 22K gold, tungsten. These three materials appear almost everywhere in quality calibers. Platinum is the most expensive and most noble choice—Czapek uses it in its own SXH in recycled form. Tungsten is the most economical solution and widely used in mid-range segments; Yema adopts it in the CMM.20 and CMM.29. 22K gold is the aesthetic choice par excellence, used by Panerai in the P.4100, by Parmigiani in the PF703, by Hublot, by Louis Vuitton: the rotor becomes a decorative surface, often guilloché or engraved.
Positioning varies. The most common solution is eccentric, off-centre: the micro-rotor occupies a sector of the plate, generally between 6 and 12 or between 3 and 9. This frees up space for other indicators on the same plane, as Panerai's P.4100 does for the perpetual calendar. Some manufacturers position it centrally, others—as one of my sources indicates—mount it perpendicular to the movement's plane, a less frequent and technically more complex choice.
Ball bearings are almost always present: they reduce friction and improve winding efficiency, partially compensating for the lower mass compared to a full rotor. Without quality bearings, a micro-rotor loses much of its energy advantage.
The practical results are measurable. Parmigiani's PF703 achieves 3.07 mm total movement thickness. Czapek SXH7.1 contains the finished watch at 4.2 mm with 60 hours of power reserve. Schwarz-Etienne claims 86 hours on its ASE 320.00. Not all micro-rotors, however, are equal: a poorly balanced small rotor or one mounted on cheap bearings charges poorly and wears quickly. I've seen pieces from the Sixties with integrated micro-rotors pass through the workshop—Büren and Universal Genève used them—and the difference in durability between a well-maintained original bearing and a poorly replaced one is evident.
A note on absence: in the Schwarz-Etienne MSE 311.00, the micro-rotor was eliminated entirely in favor of two barrels in parallel on manual winding, a solution that delivers 120 hours of autonomy. It's an opposite but equally coherent choice: when thickness is not the main constraint, manual winding with a double barrel can deliver superior energy results.