The b18c1 intake manifold has a dual runner/ dual chamber system.
The butterflies block big fat air until needed. Then both air streams meet in a small deep chamber particular to each runner. This air meeting occurs before the air is drawn down the tube towards the injectors.
I am speculating that the long runner air flow sits on top of the fat short air flow as they meet in the 2nd small chamber and proceed to the injectors. Now, I have reason to believe the big fat air is squeezed toward the injector by the pressure of the "long" air stream joining and traveling on top. Remember the long runners are always open. I see a venturi effect amplifying the fat runner air delivery.
The small channell chambers will also effect reversion waves within the runners. It will absorb the wave prior to going back up to the main intake chamber. Like capacitors absorbing certain electronic frequencies. When the bounce wave hits the small chamber the air slows quickly. This makes it easier for the incoming wave to proceed toward the valves.
Of course....I have no idea what I am saying, that is why I am free to speculate.