Just finished mounting the vacuum canister this evening. Essentially it will store the vacuum created by the pump until the brakes are applied. (The thought of storing a vacuum is rather odd. How do you store nothing? I guess it's really a canister that keeps everything else out.) Once the vacuum drops below a certain threshold, the switch at the top of the canister will kick in and start the vacuum pump again.
You can see the orange line running from the newly installed canister on the right, to the brake's vacuum assist on the left.