its a section you add the the exhaust where it acts as a silencer, but its adjustable. There is a flap which block the exhaust flow when you turn it one way, which will decrease noise level. When you need the exhaust flow, at a press of a buttont he flap turns the other way and the exhaust is back to normal flow. So during normal driving, the car can be quiet and it will be damn loud when you need it to go. Dont know if I've explained it well.