need help, hei distributor

[c20pickup]

ok had a little brain fart, and i need the answer to probably a very simple question
looking to fire up my fresh motor here in the next month and i was fiddling with my hei distributor tonight and i saw a plu with three wires going to it. for the life of me i cant remember what its for (been almost two years since i pulled the motor). i know the distibutor needs power to the ignition and 12 volts going to it constantly to provide spark, but what is that plug for? the cab is off the truck right now so how do i jump this plug with a battery in order to fire up my motor? as in, how do i make it work while the motor just sits in the frame?
i feel kinda dumb for asking this but hey i guess thats what this board is for!