Fig 1. Definition of the global coordinate system and the bat coordinate system.
The drawing represents the instant of ball-bat contact from the top view (left) and front view (right). Impact X deviation is the distance between the sweet spot of the bat (white circle; rss) and the center of the ball (rImp) in the direction of bat vector. Impact Z deviation is the distance between the sweet spot of the bat (white circle; rss) and the center of the ball (rImp) in the direction of bz.