i think this explains the two properties that some folks confuse;
Really it shouldn't be called back pressure because back pressure = loss in performance.
Basically; a "proper-free-flowing" exhaust exhibits properties which give it a "scavenging affect". Your exhaust has areas of low pressure (a vacuum) that helps suck exhaust out of the cylinders. This is only achieved if the exhaust is the correct size which allows gas to be accelerated as it exits the engine, hence creating a low pressure in the exhaust. If your exhaust is too free flow/large, it loses it's ability to affectively accelerate the gas in the exhaust and cannot create a low pressure to "scavenge" the gases from the cylinders. Headers are very important in this. Smooth bends, no cat, but proper exhaust diameter (not too big, not too small) also help in the scavenging effect. If you've ever felt your exhaust, you'll know it pulsates. Think of each pulse as a ball of energy leaving the engine via the exhaust. Trailing each ball, is a vacuum, which in turn helps suck the next "energy ball" out of the cylinder. If your exhaust is too large, you lose the "air seal" for the energy balls and cause turbulence(and turbulence causes back pressure as well), too small and you get back pressure.
hope that helps .... G.