I'm on this project myself. It's not the center of the spring, it's at the end of it, winding the spring more, providing more tension, and helping raise the window faster than it likely is moving now. I've got a great set of links at this post (plus a bunch of other door-related posts).
http://forums.pelicanparts.com/porsche-911-technical-forum/792312-meet-my-31-year-old-window-motor-winter-project.html