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Yes, you cannot think of an exhaust system as static, garden hose type flow. People use the term "backpressure" incorrectly. Backpressure added by say, putting a restriction on the muffler tip will always cause a power loss.
Ever see a bowler fly into the lane after his fingers get stuck in the ball? That's how you should think of exhaust side scavenging. The exhaust pulse compresses hot exhaust gases and sends them out the exhaust port at a high rate of speed. That's the bowler's arm putting energy into the ball.
The next time the exhaust port opens, there's going to be a vacuum right at the valve because the last pulse is still moving and creating a low pressure wave behind it. When the exhaust valve opens, this pulse actually sucks the exhaust gas out of the cylinder and brings in fresh air through the intake port. That's like the bowler being dragged into the lane by the ball. The energy he previously imparted onto the ball is now being returend to his body and he goes flying.
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