Such a system also is called a "dam muffler"
The volume of the pipe should just allow the exhaust gasses to escape, and use the header pipe gas flow inertia to suck in the fresch charge. (mousse can mufflers are a bit like that, whereas tuned pipes use pressure waves)
Ideally, the pressure build-up stops the outgoing gas rush just in time to keep the fresh charge inside the cylinder. Available power may be very cloose to a tuned pipe if set up well. and the setup has appreciable noise damping. An extra added muffler chamber with smallish stinger further reduces the noise to very low levels.