This valve is hooked up to your exhaust system (pereferably before the cat) and is opened from the inside of car above 3k rpm. The exhaust will bypass your cat and exhaust and give you a full open exhaust giving you more hp. You must make sure that you open it above 3k rpm cause you do need some back pressure to create torque at the lowerer rpms!!!!! YOu can view it here. It fits 1.3/4 - 2.25" exhaust. For an NA you dont need it bigger than this.
http://www.thepartstrader.com/upload/ACF457B.jpg

this is a pic of it.