Winched work also .. so now I really don't know what it does.. I will have to trace the wiring on the thing.
It might be just a switch to isolate the two batteries. The wire gauge getting to the switch seems to be pretty thick (low gauge number) which is why MUD members guessed either battery wire or winch wire.
The precise way to find out would be following the wires coming to a from the switch as you have mentioned.
A quick test you could do (assuming that your truck is 24V), is to flip the switch (and perhaps remove the key to be sure is "off") and then measure the Volts on the two batteries with a multimeter. If my guess is correct, then you should measure only 12V on the system.
Hope this makes sense!
-Alberto
