Below is Bour's sequence of timing for a 3 run home run he hit earlier this year. Notice his timing compared to the pitcher's delivery.  His timing mechanism and barrel tip happens later then you would think. Notice where the ball is in picture 3 as he finishes tipping his barrel. The baseball is out of the pitchers hand. In picture 4 you can see the turn of the elbow and back shoulder which initiates the actual swing. In picture 4 and 5 you can see the top arm's elbow and shoulder work down and stay tight to the body, then in picture 6 you can see the top elbow move slightly away from the body as Bour adjusts to a pitch on the outside half of the plate, if the pitch was inside you would see the elbow stay closer to the body. His timing is not perfect on this pitch but he creates "bat path" for himself by in picture 4 taking his back shoulder down and to the ball. If Bour had just taken his knob to the ball he would most likely have rolled over this pitch for a ground ball out to the right side. The swing does not happen in straight lines back to front even though the swing is taught that way by a lot of people. Extension is irrelevant. Work on getting into the right movement patterns that allow you to be in the proper position at contact. What happens after contact is just a byproduct of timing, pitch and pitch location.