Not a good article. I am an engineer/inventer and have studied hitting for a number of years. Kepler needs to look at the film of his actual stroke and more simply the picture above. Good hitters actually swing flat, meaning they swing perpendicular to their spine. The swing plane is actually on a slight upward angle relative to the ground because, as you can see from the Kepler swing picture above, the spine is at a backwards angle. The pitched ball is normally released from the pitchers hand at a higher elevation than the elevation of the bat above the ground as the bat travels through the hit zone. Therefore, an optimum swing tends to occur within the same plane as the pitched ball path increasing the time the bat travels on the same path as the pitched ball. Watching Harmon Killebrew's and Brian Dozier's swings exactly follow the path as I have described here. Attempting to swing on a path that is not similar to the path of the pitched ball as it passes through the hitting zone gives you a lesser chance of contacting the ball.