"Positive Posture" is the right way to sit (not only on the Active Chair).
Make sure you are sitting on the ball's centre, with your back straight and your feet firmly on the floor. Your thighs should be horizontal. If they are not, you can install the height adjusters or inflate the ball a bit more to rise the ball. Or remove the adjusters and deflate the ball to lower it, maintaining adequate firmness.
Maintaining correct posture is important to your health and physical well being. It helps reduce stress and fatigue and also helps prevent injury. The ball chair is a great way to develop and maintain proper posture.