Input: the green shaft to which 2 gears are fixed. Output: the orange shaft on which a block of 2 gears can slide (key sliding joint).
Move the blue bar to get 3 positions of the gear block corresponding 3 speeds (one is zero) of the output. There is a positioning device (spring ball) for the bar.
To avoid gear collision the speed change must be carried out when the input shaft stops. The operation shown in this video is for the case of very slow speed.