basically it is a way to cut the cercit
(+)==/=\ == -(-) completed cercit power flowing all is well a good path
(+)==\ /==(-) when it trips it creates a losse of power for the rest as the path is "broken"
the need for this is say you have a power serge and then is like 4k volts heading to all the stuff you got plugged in well the high volts will trip the brake and cut off the "Flow"