"I think they are to shut everything down in the case of a crash."
Or any major bump. My understanding is that it will cut off the power to the fuel pump. Because modern fuel injection systems need a very high fuel pressure to work, it doesn't need much imagination to see the effect that petrol squirting out all over the hot engine could have!
They are reset by pushing down on the rubber cap on top, I guess there is a small button under. You can feel a small but audible click when activated. I have pulled a wire off before now, When I have needed to turn the engine over, without starting it - to get oil pressure for instance.
