If you look at it closely you will see that there are two outlines, the lower one showing the car on a hook in the air.
If you press the lower power of the switch, it lights up and disables the motion alarm so the car can be towed or trailered in the event of a breakdown whilst it is locked up without the alarm going off.
Start the car and press the top part of the button to re-engage the full alarm system. The lower light will go out.