My pipe doesn't quite look like that.

Looking down from the inlet, there are two holes at the end of the expansion chamber but they are side-by-side and equal in diameter. After that follows the re-compression chamber. If you set the pipe so that the two holes are lying horizontally and then flip the pipe over to look down the end, there are two exhaust holes prependicular in axis to the expansion holes.

One smaller hole (presumably) provides the necessary back pressure constricting the re-compression chamber. The other one is larger and has a small duct which provides the actual exhaust path. In my pipe it is center offset from the actual pipe outlet rather than inline (straight shot). Dividing the exhaust in this fashion allows them to fine tune the back pressure necessary for the 45FR in particular.

Pipes kind of work like a turbine - the expansion and constriction of the flow sets up a pressure gradient and accelerates the flow increasing the efficiency and power of the engine.

What's that 45FR and 401 going in to? Mine are destined for an MK Arrow 40.

David.