Miamivice, if your car is not a 70 then it looks like someone bypassed the can and hooked the vac lines together to get it working. That's perfectly fine too as I think Ace(tylercharger69) and Jim Shine(i know Jim did) have both done this.
To answer your question, it holds vac for the operation of the headlight doors and it will allow the one time operation of the doors if the car is not running. If the lines are bypassed then you have to have the car running to operate the doors because it runs off the vac created by the running motor.
v/r
Mike
Yup, thats is how I did it. All the Chargers we have around here are done the same way. In my opinion, the relay mod for the electrical part and the elimination of the can are acceptable mods.