Economy battery cut-off switch. Most race sanctioning bodies require the use of a kill switch for easy and complete cut-off of power from the battery. The switch has to be accessible from inside and outside the car; the external trigger should be located on the windshield scuttle panel, preferably on the driver's side. It should be clearly marked by a red flash in a white-edged blue triangle.
This switch, mounted accessibly, can do the job; or mount it less accessibly and it's an anti-theft device. Comes with two (universal) keys and instructions that tell you to use it to interrupt the battery ground. If mounted in a locked boot or other secure space it should deter even experienced thieves when you use it.