It was originally suppose to be connect to the ported vacuum source which has no vacuum when the throttle blades are closed. The vacuum single will begin when the throttle blades begin to open. Sitting the timing, you want to disconnect the advance vacuum line in case the idle is higher causing the the timing to advance.

Some people choose to use a manifold vacuum source. That is the port that has vacuum single all the time at idle and when throttle blades are open. Used mostly on none stock engines. That will give you more advance at idle, often helping the idle quality and aid in a little higher idle speed and enhanced cooling at idle.