The issue isn't back pressure, it's reflected pressure waves. An untuned system will have exhaust pressure waves leaving the combustion chamber reflected back up the pipe and this is not good. A tuned system will have an expansion box positioned at a critical point in the system to act like the outside world, and pressure waves are not then reflected back to the engine. The effect is most efficient at a particular point in the rev range calculated to match the maximum engine power output.
Hope that makes sense.