930 Porsche
As others have stated, there are many different distributors out there depending on which year car. Add to that the possibility that you do not have the original distributor in your car and then you have a recipe for mass confusion. I can tell you that I have read every freaking distributor thread on this forum, as well as the factory service manuals and was confused as HELL by how the thing is supposed to work. I would recommend that you purchase a mity vac and test for yourself what happens to the timing when you apply vacuum, or pressure to each of the ports on the distributor.
I hate to add another voice to the confusion, but it appears that you have a 1978 49-state US model. Assuming you still have the factory distributor it should be the same as mine or as Fredmeister's. I found this thread to be particularly enlightening:
http://forums.pelicanparts.com/showthread.php?t=400685
Check out in particular the posts by 356-930 as he is using a US 1979 engine so should be the same as your '78.
I can tell you with absolute certainty that my distributor has no vacuum advance, (only vacuum retard) and does have boost retard, and behaves exactly as described by 356-930 in the post I mention above. I know this because I have tested it myself.
In my case, port 1 (on the outside of the can) is plumbed to the TB and sees manifold vacuum. I would also note that there is a thermo-time switch between the distributor and the TB. The thermo-time switch blocks the vacuum signal to the distributor for 15-20 seconds at start up, effectively advancing the timing and increasing the idle speed for a few moments. With vacuum, the timing is retarded considerably (~20 degrees). When manifold vacuum drops upon opening the throttle, the retard goes away effectively advancing the timing. Applying pressure into this port has no effect. Port 2 (on the inside of the vac can) is plumbed to the boost recirculation housing. Once upon a time, it was hooked into some gizmo's related to the air injection system (long gone on my car and most others probably). Pulling a vacuum on this line has no effect on timing, while applying pressure will retard the timing.
Only a shade-tree mechanic here, but this is what I have observed with my own eyes very recently.