it changes the way the sub moves... if at 0 it moves normal... if at 180 when it hits it does reverse than it would if it was normal..id leave it at 0, but its a "ear" type deal
The point is to help you get your midbass in phase.
I switched mine to 180 degrees out, and the bass was instantly less obviously emanating from behind me, along with the the midbass being much more pronounced.