That's a built-in bimetal circuit breaker designed to prevent complete loss of lights in case of an overload or a short circuit. The lights will flash on and off enough to provide a little bit of illumination.

If high current headlights like some of the very bright halogen units are used, their normal current draw might be enough to cause the breaker to cycle. That's where a set of relays need to be incorporated into the circuit to take the load off the light switch.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!