Ball flight curvature is directly related to the orientation of what TrackMan has labeled the "spin axis." A positive spin axis will curve the ball to the right, while a negative spin axis will curve the ball to the left.
There are three components that have to be considered when trying to explain ball curvature or what we refer to as spin axis: club face to club path relationship, off-center impact, and club face rotation rate through impact.
The primary source of generating spin axis is explained by any difference between the club path and the angle of the club face to the path. For right-handed golfers, an open face (to the right) relationship to the path or direction of the club, will produce positive spin to the right or what we call a fade. Negative spin axis to the left or a draw, will be produced by a closed club face relationship to the path.
Off-center impact occurs if the ball is impacted anywhere but in front of the center of gravity of the club. If off-center impact occurs, the club head will tend to rotate during impact. The counter action of the ball is to rotate the other way, like a gear. This added component of spin will add or subtract from any spin axis.
A rotating club face through impact can also create the counter action of the ball to rotate the other way, (like a gear), exactly as explained above for off-center impact. It is primarily the speed at which this action occurs, that has an effect on spin axis.
Understanding spin axis, or what creates ball curvature, is an important part of becoming a more educated player. This knowledge can help us better assess our ball-flight and personal patterns, which then leads to improvement.