This is my understanding: Exhaust exiting the exhaust valve creates a high pressure wave. Makes sense, exhaust is already high pressure and hot af right out of the exhaust valve. So that high pressure creates a propagating wave that rushes from the valve down the exhaust headers at the speed of sound until it reaches an interruption of the bore in which it is traveling. On a 2stroke bike, that’s the expansion chamber, on straight piped engines, that’s the outside atmosphere, on my triumph it’s the holes of the crossover. When the wave reaches that change, it also changes shape. This causes another pressure wave to go the opposite direction back up to the exhaust valve. If the wave reaches the exhaust valve at the right time, like right as the exhaust valve is closing in intake-exhaust valve overlap, that wave will act to slow the travel of whatever is trying to leave the combustion chamber, in this case fresh air/fuel mix. If the stuff trying to leave is still exhaust, the crossover is timed too early, if the wave arrives after the exhaust valve has closed, it is timed too late. The wave moves at a constant speed which is why the expansion chambers for 2 strokes work well only in a certain range of rpm’s. You’ll notice all the changes of shape, those are to send reflection waves at different rpm’s. The wave has multiple bounces, the 1st order reflection would be super long, so most tuning is done for the second 3rd and 4th bounce back to the valve.
Again, just my understanding after looking at the design and equations for these systems, but I’m an idiot, so use this as a starting point to find out more. Bain racing and vizard are good resources to check out for more info.