Input: green shaft carrying three gears.
Output: pink hollow shaft in which orange shaft slides. The pink and orange shafts rotate together owing cyan key, that has a revolution joint with the orange shaft.
Red, yellow and blue gears engage with the green gears and idly rotates (with different speeds) on the pink shaft.
Depending to axial position of the orange shaft which is controlled by violet crank, the cyan key enters into key slots of one of the red, yellow and blue gears and connects it with the pink output shaft. Red pins help retreave the cyan key from the gear key slots when the orange shaft moves longitudinally.
There is a flat spring (not shown) that forces the cyan key towards the gears.
The video shows 3 positions of the orange shaft that give 3 output speeds.