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.
As we must account for every idle word, so must we account for every idle silence." ~Benjamin Franklin