The <span style="color:yellow">yellow</span> wire is only an inline fuse holder and could have been used for anything previously. Whatever it was connected to the previous owner didn't want you to have along with the vehicle.
I don't think it has much to do with the operation of the switch as it's connected to the other battery not the one shown. It's connected to the main power wire I'm guessing it could have been used for a small air compressor, mobile phone charger... any of those sorts of things.
Think the override would be for if the main battery is dead, push the button and start off your aux rather than look around for a victim with your booster leads in hand. Only way to figure it out is get the multimeter out and poke around. Test the theory by disconnecting your main battery, push the button and see if it wind over.


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