Hi Paul,
When you are under "performance conditions" vacuum is low and vacuum advance is not advancing the ignition, just the centrifical advance is working as RPMs increase. This the same as having a centrifical only distributor. When you get back to high vacuum conditions, you are off the throttle. The vacuum advance puts more advance into the distributor cleaning the plugs and increasing fuel milage. This is truely the best of both worlds - at least until you go for computer controlled ignition timing.
Jim
P.S. Centrifical advance is what retards timing for starting conditions.
