When the cars is moved without the movment sensor being disarmed its activated and then alerts the tracker company that the car has been moved, possibly illegally, dependent on which system you have thay will either contact you by text or phone and confirm if the car has been stolen, if it is confirmed by you the car has been moved illegally, they will alert the police, in the relevant area, who will be able to track the car to a location, clever stuff really, its been used on comany vehicle for years especially cash handling companies.
it makes a huge differenc to your insurance too. There is a thing with some of the companies that they guarentee to get your car back, check the TRACKER web site, well worth the money IMO