It's simple. Youtube D-plane.
Here is a long Manzella video that goes into detail.
When the club is moving down, that adds an outward component to the path. So a swing that is moving perfectly inline with the target but down towards the ground will actually be slightly in-to-out as it effects the curvature of the ball. In Trackman terms, this means a swing with a 0 degree face angle and 0 degree path will actually result in a slight pull draw.
When the club is moving up the opposite is true. A out-to-in component is added.
So in practice, to hit an iron perfectly straight, your face would need to be a 0 degrees to target and path would be out-to-in (left) a couple degrees to counteract the issue we are talking about.
To hit driver straight, when hit on the upswing, the path needs to be a couple degrees in-to-out (right).
This is relatively high level stuff that most people shouldn't worry about. Talking about minute differences really. And without Trackman you would never know to the specificity you needed to anyway to figure this out.
Last edited by Your Boss; 10-29-2011 at 01:32 AM.