As the #2 starts getting prespooled it beings to build boost. If the boost gets high enough before both valves that control #2 open fully, the reed valve serves as a bypass and lets the boost start flowing into the intake path. Basically a pressure relief as #2 comes on line with #1. Sits on top on the passenger side of the engine in the intake path. You can see it sinces it's inside the piping. Only the cover to it can be seen.