EVELO Electric Bicycles have 3 modes of operation; pedal assist, throttle only, and pedal only.
In pedal assist mode, there is a sensor on the crank that activates the motor once you start pedaling. The amount of "boost" you get from the motor is determined by you-- it can be either "low", "medium", or "high". Several seconds after you stop pedaling, the motor turns off, and power is cut immediately if you squeeze either brake lever.
You can use the motor by itself by simply press the throttle on the left hand grip-- you don't need to pedal at all.
Of course, you could leave the electrical system turned off all together and just it pedal like a regular bike.