The Speedometer Cable, powered by the Driveshaft, turns as well. The cable spins a Magnet around at the same speed inside the speed cup. The Magnet rotates continually in the same direction (in this case, counter-clockwise). As the speed cup turns, it turns the pointer up the dial, indicating the Vehicle's speed.
The Odometer shows the distance your vehicle has travelled, while the Speedometer show how fast your vehicle is going.