It is a sophisticated cycle computer that measures power generated by the rider in watts. The hub has strain gauges and other internal bits that measures torque, speed, distance, watts, time and estimates cadence and sends the info wireless onto the CPU. I purchased the cadence sensor to get real cadence. Instead of training by heart rate I train using established power zones that are similar to heart rate zones but are more accurate because power generated is consistent where HR can vary due to heat, fatigue and other factors. I had to do specific testing to determine the lactate threshold heart rate and average watts produced over a specific time frame to generate the data needed to establish the power training zones. It reads complicated, and to me it is, which is why I invest in a coach and spend my time riding and following directions.