Certain factors have to be right for the system to work - car must be up to operating temperature (i.e. won't usually work in the first 15 mins of a journey), and it won't work if there's a high current load at the time (i.e. if you're running xenons, and the stereo, and the heated screen, aircon, heated seats etc)....
Basically, the car will switch itself off when its up to temperature, and able to sustain all the systems you currently have running from the battery, and still have enough power in reserve to re-start when you depress the clutch
