| Exhaust systems and exhaust effects are a heck of a lot more complicated than people realize. reversion waves are the result of expanding energy leaving the exhaust valves as they open, like an expanding wave of an explosion. this wave moves at like 1500 feet per second. Behind this wave is another one, formed by spent combustion gasses moving at like 500 feet per sec. these waves clash as the faster one meets the slower one, and the waves become "reversed" back up the pipe. This "reversion wave" can affect the power that the motor is capable of producing by introducing spent gasses back INTO the combustion chamber through the valves, and causing pressure blockages and turbulence in the header.. Mufflers and exhaust systems are designed to attempt to eliminate or minimize this condition. The full explanation is a lot longer than this, but hopefully you get the idea. | 
