Not to nit-pic but I don't want anyone to be confused. Cadence, in cycling, is how quickly you are turning the cranks. Not how quickly the wheels are spinning. You could be coasting with a cadence of 0 but you could be traveling 30mph ie., coasting downhill. There is usually a sensor mounted on the chainstay and a magnet on the pedal crank. Each time the magnet passes the sensor it counts on revolution. RPMs are then displayed on your computer.