In a useful way, this mechanism simplifies the task of synchronizing a watch with an official time signal: as soon as the crown is pulled to set the time, the movement stops and the seconds hand automatically jumps to zero, i.e. at 12 o'clock.
This function is handled by an intricate system of levers. The last element is a zero-reset lever, which presses against a heart-shaped cam disc. The seconds hand is located on the same arbor. Therefore when the heart-shaped cam disc is swung to its initial position by the zero-reset lever, the seconds hand simultaneously jumps to zero. When the crown is pushed home, the movement starts running again, as does the seconds hand.