So, when you hit something kinda hard and your display starts flashing random errors and the climate control says "err" and your car won't start and you're somewhere inconvenient and you pay for a tow, it'll be the inertia switch and the jaguar dealer will charge you $80 to press it.

Of course, saying something like "inertia switch tripped" would be too difficult, it has to make you think you've ripped out half the underside of your car instead. :/