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!

[mmiddle]

The 3-wire plug coming out of the base of the Dizzy plugs into the cap. The "BATT" terminal gets switched power. The "TACH" terminal is just that.

To run it in the frame.
1) run gas to fuel pump
2) hook battery POS to big stud on starter
3) hook battery NEG to engine block
4) hook temporary switched battery POS to Dizzy "BATT" terminal
5) hook temporary switched battery POS to small outside terminal on starter.

Flip ON Dizzy switch. then flip the starter switch. It should run

[c20pickup]

really? that easy huh? thanks!!! cant wait to fire this bad boy up! almost 2 years in the making!