It cuts down some of the induction roar and improves the throttling, normally the first (closest to intake) is open slightly ahead of the main one so the main one on the plenum retains its electronics the second one is bare and is just a t/b with any other ports blanked off
I did my research on mx5 nutz where there is a wealth of info and then adapted it to fit on my Haynes all out of site
Plenty of comments as people ask what the noise is and a smile when you tell them, David said it was like being chased over the mountain by a monster lol