Its the "Maintenance disconnect button" and there is no (Australian) requirement for it to be within reach of the driver,(under the bonnet is OK). I installed mine in a position to make use of the old fuel switch hole & its a bloody nuisance as I occasionally hit it with the heel of my left foot & am left wondering why I've lost all power.
hitting it in an emergency is NOT a good idea as you will lose Regen braking. Silly having it in reach of an idiot driver I think. (that includes me)
http://goingbush.com/AULRO/button.jpg