What you have is a Perko battery switch. Perko is a well established marine equipment company. Some models of switches WILL NOT interrupt the field current (increasing the possibility of damage to the alternator) if turned "OFF" with the engine running. Newer switches can be turned "off" or can be switched between batteries (if a 2 battery selector type) while the engine is running without fear of destroying your alternator. Lookup your model # on the Perko site and download the instructions for more info.
http://www.perko.com/catalog/category/battery_switches/
