Thats the aux shock, it gives the bike extra travel, buddy says he likes this bike better than his operator for pretty much everything. except for race runs.
(Nearly a year later) That's the magic link. To improve pedaling efficiency it only uses the air shock when going uphill or on flat but once you start going down, the force on the bike causes it to go though the aux spring. It's a 120-200mm bike. Pretty sweet sounding but I've never ridden one.