Help Understanding Performance Exhaust

I have two Civics. The 96 EX D16Y8 I'm tuning has Airmass headers on it, factory cat, and a killer carbon fiber muffler & tip. The other 95 EX I'm using for parts has a full Magnaflow system from the header back. I'm thinking of modifying the 95 model Magnaflow to fit the 96 civic, leaving only the Airmass header. What I'm not understanding is what the cylinder after the cat converter is or does. Can someone explain this to me? Here's a shot of what I have:

http://www.planetmagnaflow.com/shop/magnaflow-15641-40.html

http://www.alatrails.com/photos/albums/userpics/10391/muffler.jpg (http://www.alatrails.com/photos/albums/userpics/10391/muffler.jpg)