In stock I like to run the batts forward to give me more steering (I like ALOT of steering, lol). In mod when I want the car to be more consistant and easier to drive I will move them back, since it will give the car a more planted stable rear end.

Its basically a way of enducing a handling charactistic without changing the suspenion. In full scale racing, espically f1, teams always try to build the lightest car possible so that they can put the weight (as for the car to be legal) wherever they want. The Rev. 3 allows you to do this.