The theory is; after the gases leave the head, and get 6" out, then there not that much back-pressure, as the engine want's some back-pressure.
This ought to stir up opinions, lol.
I tend to agree with the backpressure theory.i had a 340 valiant that ran better with the exhaust on than with open headers. i guess the engine was trying to tell me something. bigger is not allways better.