Two types of sensors can activate electric assistance on an electric bicycle: the cadence sensor which determines when the user pedals and the torque sensor which determines how hard the user pedals.
For example, at Quantum, the Broadway model uses a cadence sensor which activates the electric motor when the user moves the pedals, giving an electric assistance very similar to that when using an accelerator (throtle).
Other models like the Stromer use a torque sensor, which gives electric assistance by activating the motor when there is pressure on the pedal.
Finally, there are models, such as those equipped with a Bosch system, which use a combination of the two sensors to provide a smoother and more comfortable assistance.