One way yes. But some air does flow through a muffler in the opposite direction of exhaust flow during the 2nd half of the exhaust phase of a 2 stroke, it's just the way they work. A steady stream of air has a resistance relative to the friction of the muffler, but when the air alternates many times per second the volume of air (length of the muffler/pipe) begins to affect how the engine breaths. This is the entire theory behind how tuned pipes work. They're designed to cause just enough back pressure at a certain frequency to help blow-back the tail end of the exhaust; which contains highly mixed fuel and air giving a torque boost at the frequency the pipe is tuned for. It's analogous to how a capacitor works in electronics. 4 inches of half inch wide tubing doesn't contain a massive amount of air, but it's still air that has to be accelerated. Constant streams of air aren't subject to this because only resistance of the muffler walls affects drag. Acceleration or deceleration of a mass ALWAYS takes energy. Take the same straw and do the same thing trying to fill the straw with water and then blowing it back out as fast as you can, double the length of the straw and do it again. A human has to use water, because the frequency of air in a 2 stroke is much much higher than a human can reproduce. Keep in mind this is just physics, I don't know how much it affects the actual power output of the engine I just know it does.