Does anyone know how the vacuum diaphragm on the distributor works?
One side is to retard the timing correct?

does the other side advance it as the vacuum drops or changes?

Should I have noticeable vacuum on that hose when I pull it of the diaphragm?

See post 18 in this thread.

From the Cap'n so it is spot on. Best explain agin I have seen.

My only problem with the Cap'ns explanation is that the same size ports on the throttle body are connected. I followed his direction and connected the same size ports and that means I have no vacuum advance, only vacuum retard. I went with the expert but still wonder....

His post makes sense as to what I am seeing.

I did too but it seems to work.

The 74 cars, and some later ones, do not have vacuum advance. They have the fitting on the distributor dashpot, but not the one on the throttle body. So there's a good chance the setup that puzzles you is the stock setup.

--DD

As I see it, you will get less total advance but end up at the right spot in the end. You start out with a bit more advance at lower RPM's, not including idle.
I thought it was odd to have the Vacuum advance only to ignore it but I do as I'm told.

The vacuum advance helps with off idle stumbling.

At 3500 rpms, and even before that, the mechanical advance starts working by centrifugal force, so as long as both springs are not broken, and the mechanism is working, you get total advance in the end.

Vacuum retard helps with run on which is when you are coming to a stop light, the idle takes too long to come back down....when vacuum retard is working, it snaps the advance plates back to zero....so you don't have to wait.

You can run without either of them.
You tune the car without either hose attached.

I have had mixed results tuning with the retard hose....but I was tuning a car for autox and we did not Ike it coming back to idle so quickly...it performed better with some advance still in play between shifts.