In fact the brakes aren't involved at all but it makes for a good title. Engadget is reporting that OnStar is adding a new feature in 2009 that will pinpoint a vehicle via GPS when the owner reports it stolen, and then when the police have it in sight OnStar "interacts with the powertrain to reduce engine power, which will [in turn] slow the vehicle down gradually." Seems like a safer option than jamming on the brakes and launching the thief through the windshield, but then again that might be a better deterrent!
