here is some input from another mechanical engineer.
im don know if im just reiterating previous statement but, it needs to be explained in general terms for people to understand. at low RPMs , backpressure needs to be built for there to be any movement of the system. backpressure is built up until your pistons begin to move due to these pressure forces. if you have too large of an exhaust diameter and not enough air flow in, there will be no pressure build up at low RPM's resulting in less power in this range. a larger exhaust is only necessary when you are increasing the air flow into the head. for NA this would be adding a new intake manifold air intake cams. for turbo your exhaust size definately needs to be increased for max efficiency depending on the size of the turbo. and of course the other force that adds to cylinder pressure is air/fuel mixture. most often you need to change the air fuel mix in a car with exhuast and headers because combustion is not at peak efficiency, any more. backpressure is mainly a sign of how well your system is preforming with all the other components combined.
simply stated, if you are wondering what a backpressure exhaust is doing, it is allowing the exhaust to move more easily through the system without sacrificing the all important BACKPRESSURE.