I've noticed that some of these cutouts are in the shape of a 'y' but I'm not sure how it works with these flaps. If there is just one, and it opens up the extended part, then the exhaust is flowing through the extended part AND the rest of the exhaust pipes. If there are two, then it should close the rest of the pipes and make ALL of the exhaust shoot out the extended part. So how exactly do these work?

 

One butterfly valve. The exhaust gas will take the path of least resistance. When the valve is open 99% of the gas leaves through the butterfly valve.