Pull and release brown tow to let yellow ratchet disk turn 180 deg. One end of the tow is fixed to blue disk. Orange leaf spring keeps the yellow ratchet disk immobile during its dwell. A circular slot on the blue disk and a pin on the case limit oscillating angle of the blue disk. A coil spring (not shown) makes the blue disk turn back when the tow is released.
Replacement of cable drive with rack-pinion one is possible.