It means that the window will be centered under where your cursor currently is. If you're having trouble activating it, try drawing small circles with your cur

It means that the window will be centered under where your cursor currently is. If you're having trouble activating it, try drawing small circles with your cursor, rather than just shaking back and forth. Some people find that easier. (That being said, I developed it on a Magic Trackpad, so the shaking does work -- it just depends on your personal mousing style). I hope this helps. :)