The motor assistance kicks in as soon as you start pedaling and have gathered a little momentum. The motor assistance of the ST2 adapts itself to your riding style. The harder you pedal, the more assistance you get from your motor.
For your safety, the motor of the ST2 does not start to run until you have reached a minimum speed. However, always keep a brake engaged before placing the first foot on the pedal. The strong acceleration takes some getting used to. Uncontrolled takeing off can lead to a crash or to serious injury in traffic or on a loose surface.
Higher motor-assisted speeds and the additional weight may take some getting used to. Practice the operation of and riding with your ST2 in a calm and safe place before using it on public roads.