Double locking helps prevent the unwantables breaking into your car.When activated (by pressing the key fob lock twice) the car door will not be able to be openned from inside. i.e. if someone breakes your window to get in they wont be able to open the door by leaning in, they have only one option and thats to climb through the window. By the, way double locking can only be activated when the car is not being driven, this is because in the unfortunate event of an accident and you are trapped in the car the window can be broken and door openned to get you out if needed.