Input: green shaft with four gears fixed on it.
Output: pink shaft, that has a cylindrical joint with blue angle crank.
Yellow gear has sliding key joint with the pink shaft and engages with orange gear. The blue crank carrying the orange and yellow gears can move with them along the pink shaft.
To change speed:
1. Pull violet positioning trigger, turn back the blue crank and move them to other green gear.
2. Turn forwards the blue crank until the orange gear get in mesh with the green gear and release the trigger.
A red flat spring forces the trigger towards positioning holes.
The video shows the change from highest to lowest speed.