x and h pipes

With the X it's more like as one side flows past the X, its inertia creates sort of a "vacuum" that pulls the exhaust from the engine out more quickly from the other side. I imagine the H does the same thing, just with less effectiveness because each side doesn't flow right past the other one side-to-side like the X; it has to sort of "reach through" the connecting pipe and overcome those sharp 90* bends.