They work magic, when you want to have a conversation with a girl in the car casually driving you just close it down, when you see a cop just close it down and your exhaust will be quiet. When you open it back up its like its not there at all and you can tear it up once again.
Obviously your not going to have power when your closing it down, the whole point of it is to quiet down your car, if you want power you open it back up and rip, quiet and powerful dont come hand in hand often.