The Flat spot
The Length of the Slide plus the amount of Knee continuation equals the length of an individual’s Flat Spot.
I have always equated the flat spot in bowling with that of the divot taken after striking the golf ball with an iron. Both are the result of proper technique……not a learned technique in itself.
In the first slide Doug’s foot is flat on the floor and he is about to begin his slide. If this were the beginning and end of his slide, the swing would follow the red arc defined by the swing center (white star #1) with minimal flat spot. The small amount of flat spot would be the amount the shoulder (radius point of the swing) would move forward in order to deliver the ball.
In the second view his slide is now complete and the swing radius has moved forward as a result of the slide foot moving from the yellow to the green line. This movement of the slide foot forward also moves the radius of the swing towards the foul line to position #2. Because the radius of the swing moved forward the ball path can no longer follow the original red arc. It is now on the path of the blue arc.
As the shoulders close and the body weight transfers to the slide foot the radius of the swing moves forward to position #3.
The body weight continues to move forward and so does the radius of the swing, now at position #4. From position #4 the swing will arc upward on a path very similar to the original red arc.
My summary: The length of the radius of his swing (shoulder to ball) never changes. The radius point of the swing moving forward as a result of Slide distance and Knee Continuation creates the length of his flat spot (orange line).