It is a mechanism invented by A. Lange & Söhne and now also offered by other manufacturers. It serves to instantly bring the seconds hand to zero when the winding crown is pulled out to reset the time. Its operation is based on a system of levers activated by a bascule: when the winding and setting crown is pulled out, a first lever makes contact with the balance crown, blocking it, while a second lever brakes the seconds wheel; at the same time, the same bascule activates a third lever, which presses against a heart-shaped cam to which the seconds hand is connected. When the crown is pulled out, the heart-shaped cam is pushed to rotate to its initial position by the lever system and the seconds hand connected to it is set at zero. During the fast rotation of the hand due to the action of the zero-reset mechanism, the seconds wheel is held still by the brake lever, while during normal running of the watch, the hand and seconds wheel move together. When the seconds hand rotates by the action of the zero-reset mechanism, while the seconds wheel is held still by the brake, the spring connecting the two slips on the surface of the wheel, breaking its connection. When the crown is closed, the movement is activated again and the seconds hand resumes its rotation. In this way, the watch can be set to the second, on a reference time signal.
