If you have ever taken a physics class, its not hard to figure out.

A stiffer rear bar reduces body roll. Body roll causes a outward momtentum when going through turns, thus pushing the car towards the outside of the turn. This is easiest to demonstrate on snow, when the back just slides out.

actually, I dont think i can do this explaination justice. Im gonna let some one else take over. . . sorry bro.