RE: How do the transmissions work?
It doesn't have as much to do with engine RPM as with axle RPM. On a normal 2-speed there is a cam attached to the spur shaft, on which the 2nd gear spur rides. When the car accelerates and the wheels are already going up to the top of the first gear, there is a little finger inside the cam that comes outward and catches the second spur gear that rides on it. the thing that prevented it from coming out before is a little spring that you can adjust with an allen wrench. Some 2-speeds have clutch shoes instead of a finger like protrusion for smoother shifting. 3-speeds? Same principle, just a little more complicated.