If the engine can stand it at low engine speeds without detonation, it allows a more closed idle set up and helps improve mileage a little. Ported applies vacum at some point above idle position where as manifold is there at idle.

If you try it and get a little pinging, need to adjust the vacum so less vac advance available, use allen wrench and turn adjustment CCW in vac diaphram on distributor a turn at a time and keep track of it.

The Holly has a seperate base plate. There is a passage way that leads from the bottom up through the base plate into the main body that provides a manifold vacume signal to the power valve.

In the case of a back fire, it will also move pressure spikes and these can rupture the thin rubber diaphram in the power valve and then it allows gas to be sucked through it by the vacume signal.

The port isn't intended for gasoline. When vacume is applied to the power valve it closes, when vac drops, the valve opens and fuel is allowed through metered passageways to the venturi .... when it's working properly.

The modification involves partially drilling the base plate in that passage way from the top side down just a short ways and dropping a small spring and ball in there from the top. Then, vac still pulls the ball down against the spring to supply the vacum signal, but a backfire drops vacume and the spring pushes the ball up against the main body and the pressure spike is stopped there .... thus saving the powervalve.