Pull and release the brown tow twice to let the green coulisse move forth and back. One end of the tow is fixed to the blue disk. The orange spring ensures that the yellow ratchet wheel rotates in one direction. The circular slot on the blue disk and a pin on the case limit oscillating angle of the blue disk. A spiral spring (not shown) makes the blue disk rotate back when the tow is released. Another spring (not shown) always forces the pawl toward the ratchet wheel.