Tony
The inertia switch is a mechanism to shut off the fuel supply in event of a crash and to prevent a potential fire when the fuel pump is still active continues to feed highly combustible petrol into the hot engine bay possibly now containing fractured fuel lines.
Although it is designed to be triggered only by a sudden deceleration (as one would anticipate in a collision) but it is not unknown for it to be triggered by lesser events eg vey bumpy roads . In fact it is sometimes one of the "go-to" check points if the car is suddenly electrically "dead" . I must admit I did not know there was a dashboard warning light.
