True... but it's not back pressure causing that scavenging and return of fuel/air mixture to the combustion chamber, as shown in the animation, it's the reflected pressure wave from the expansion chamber due to an increase in the resistance to flow at the rear conical section (a reflection due to an impedence change for you engineers) . And I doubt that anybody thinks that the HT exhaust/muffler is in ANY way designed as an expansion chamber. It's simply an exhast pipe and muffler that acts as a resistance to flow (causing some back pressure) but certainly not tuned to return a pressure wave.
Interestingly.. i wanted to move my muffler back to the rear wheel, so i got about a 3' length of 3/4 ID silicone heater hose, cut the stock pipe just below the bend and clamped on the heater hose. I then routed the heater hose back to above the kick stand where it's clamped to the stock muffler which is then mounted to the left rear wheel stud.
I really expected my performance to decrease due to increased back pressure.. I was pleasantly surprised! Suddenly my torque went up significantly, the high RPM sputtering stopped and the engine sounds more like some of my expansion chambered 2-stroke motorcycle engines. I wonder if it's just the additional back pressure, or whether the rubber tubing actually expands and contracts during each pressure pulse, returning a pressure wave of sorts to the exhaust port as a tuned chamber would? Kind of like an active expansion chamber? I may do a few experiments to get more information. Regardless, the engine's performance and 'smoothness' has improved across nearly the entire RPM range. Could I have a new idea here? I'm going to replace the heater hose with thinwall conduit with a similar ID. If the performance improvements are still there, i think i can just chalk it up to increased back pressure.. if they go away, i think the rubber tubing might just be a great accidental find and, of course, i'll go back to it!
Anyway... without at least a reasonable amount of back pressure, you will be exhausting a significant amount of the fuel/air mixture, reducing fuel economy, increasing the heating of the exhaust system and increasing emissions.
Good luck with your build!