This intricate clear acrylic model assembles into a fully-functional two-speed transmission. It operates on the same principle as a car transmission: all the gears are always fully engaged with each other, so you can never actually "grind the gears". Shifting from one gear to another happens when a clutch plate in the middle of the mechanism is slid to one side or the other, engaging with clutch plates connected to either the small or large gear on the output shaft. (These "dog clutches" are not to be confused with the main foot-operated clutch found in manual-transmission cars, which is a completely separate mechanism.)
The model, including all its gears and clutches, are made of transparent colored acrylic, so you can literally see the teeth of the clutches engaging and disengaging, both from above and from the side, right though the transparent gears. From these viewpoints the operation of the device is...transparently obvious.