Input: green shaft with two green gears fixed on it.
Yellow shaft is fixed immobile. Blocks of two pink gears rotate idly on the green and yellow shafts. These blocks get rotation from the green shaft in a meandering manner.
Output: red shaft, with which grey crank has a cylindrical joint.
The grey crank carrying the red, orange, yellow and blue gears can move with them along the red shaft.
The red gear has sliding key joint with the red shaft and engages with orange and yellow gears. The yellow gear is in mesh with the blue gear.
To change speed:
1. Pull violet positioning trigger, turn back the grey crank and move them to other gear of the green shaft.
2. Turn forwards the grey crank until the orange gear (or the blue gear for reversing output direction) to get in mesh with the selected gear and release the trigger.
A red flat spring forces the trigger towards positioning holes.
The video shows the change from forward highest speed to reverse lowest one.