Gravity pulls the ball down and it centers at the low spot whenever the G component is through the vertical axis. If you are slipping or skidding, a lateral G component pulls the ball to one of the sides.
Another way to think of it is think of a ball hanging on a string from the roof of your car. It hangs straight down, but if you apply lateral G, it will swing one way. If the slip indicator was flat, it would be as if that ball in your car was on a flat plate instead of hanging from a string. It would roll to the edge of the plate in a turn and not return to the center.
Last edited by zondaracer; 2nd June 2011 at 04:10.