How does emergency light switch work?

An emergency light switch uses a relay. A relay has an 5 terminals. The firs two terminals are connected to a flowing current (current flows when there is a normal electricity) The next 3 terminals are the common, the normally on, and normally off.

One of the battery terminal is connected to the common, and the other one terminal of the battery is connected to one terminal of the light bulb(which has two terminals) then the other terminal of the light bulb is connected to the normally on terminal of the relay. When current flows to the first two mentioned terminals of the relay, the circuit of the battery and light is cut off or not connected. But when the current is off, this will switch on the normally-on terminal thus current will flow from the battery to the light bulb, that will shed light. ;) on an emergency situation.

note: The normally off is not use. And may be use for some purpose.