In a 2-stroke engine, the exhaust is blown out of the cylinder by the next fuel/air mixture. Usually, a little extra mixture exits before the piston closes the ehaust port.

When you tune an instrument, you are trying to get a specific frequency to make a sound. When you tune a muffler, you are also looking for a specific frequency, but it has nothing to do with sound.

What you are trying to accomplish is getting the pressure from the exhaust to hit the back of the muffler, and then bounce back, which forces that little bit of extra fuel/air mixture BACK into the cylinder.

This is why tuned pipes have a sleeve so the muffler can be slid forward and back to get the distance just right.

When the timing is just right, the "Pipe" is "Tuned"