Bingo creepy - you beat me to it.
I'd read these were massive savers for young drivers.
You can restrict them to daytime driving for example, or very low mileage, or pay as you go.
The ones i've seen monitor your speed, braking and cornering Gs etc so they know if you are 'smooth' driver.
You pay normal at first on a shorter premium that reduces if you drive well in the previous months.