The difference between the speed sensor and the torque sensor: The speed sensor measures you pedal cadence. So it only knows if you are pedaling or not. If you pedal harder, the bike still only goes one speed. The torque sensor knows your cadence AND also the force you put on the pedals. So when you pedal light, the motor gets less power, when you pedal harder, the motor gets more power. The feeling is superior, the bike feels more alive.