Timed or manifold vacuum advance?

manifold vacume is for a crummy running engine that has idle issues be it from a lumpy cam in where the tuner does not know how to modify the advance curve to maintain the high initial required to idle a big cam, or because the motor is mechanically failing/worn out.
Chevy had there own ideas about it, but look at chevy...they were the 1st mandated by the federal government to use smog pumps, chrysler was the last.

see when you are using manifold vacuum, you are engaging the vac advance at idle, but when you dip the throttle....the timing drops off to where ever the distributor initial is set to.
The timed port will activate the vac advance only when the throttle blades are opened beyond a certain point.
Its a band aid or the latter to welding up the slots [but only in the aspect of idling] cause with a welded slot the timing won;t drop off and you WILL make more power.