"Negative back pressure" is that an oxy-moron?
"Less back pressure is always better." so why don't we all take off our headers and throw them away?
"Negative back pressure is best, and that is the goal of a correctly designed exhaust system where the scavenging effect of the sound pressure waves produces a negative pressure at the exhaust port when the valve opens."
Ahh, now we're getting down to brass tacks.
Waves, that's why. A properly designed exhaust system induces pressures waves.
And why is supertrapp best? cause it allows adjustment of the frequency and timing to match the frequency the engine uses.
In other words, for best performance time the maximum negative pressure to the closing of the exhaust valve across the rpm range.
There, do that with a coffee can and you can make a million.