heres a video of a very intersting itb set up
http://www.youtube.com/watch?v=TGvGgTS4 ... re=related
from the looks of things it seems that 1 and 4 are connected together and 2 and 3 are connected together similar to an exhaust. maybe it works in a similar fashion to the scavenging effect.
first time ive seen something like that.