This works because when the exhaust valve opens, the temp is 700 degrees at up to 70 psi, but the interval is small. As it passes back the interval becomes closer to continuous, the temp drops and the pressure evens out.
This is where I am reading Phil Smith's work very closely. He talks about an effective exhaust manifold/collector as providing as little interference between one exhaust port and another. This is the benefit of splitting a 6 into two completely independent systems. With a 2" short header-collector dumping into 1.5" head pipe the 3 exhaust pules sharing the collector should effectively line-up behind each other and head for the exit . . .
To your point "as the temp drops and pressure evens out" this system has insulated head pipes and presents smaller diameter mid-tail pipes in effort to retain velocity in a constantly cooling and rarefied collection of gases.
