There are two parts to the exhaust event: the evacuation of slow-moving exhaust gasses and the escape of pressure (sound) waves moving at up to 1,700 feet per second. The exhaust gases travel down and out the pipe, but as the pressure pulse moves through the exhaust plumbing it encounters various design features that influence its behavior in very interesting ways. A change in pipe diameter will affect its velocity, and it will also reflect part of the wave back toward the exhaust port, like ripples in a pool ricocheting off a wall. Not only that, depending on the change in the pipe’s cross section, the wave’s sense may reverse from a compressive (positive) force to an expansive (negative) force. If timed right, this negative pressure wave will arrive back at the exhaust port and help scavenge the cylinder of residual exhaust gasses as well as help draw fresh fuel charge into the cylinder, improving power and fuel efficiency. Two-stroke expansion chambers are an example of this tuning technology taken to the extreme.