The flat spot is the area in the red circle in which the blue ball track flattens and the delivery happens.


(English cc/subtitles available for the text shown in the video)

This is a translation from German of the text that goes with the video:
Players who are able to make your delivery with a flat spot have the advantage that you can benefit from a margin of error in the delivery. It has also been shown that these players also have a higher consistency of the delivery.

The following elements are necessary in order to be able to produce such a flat ball track (or flat spot)
- the inclination of the upper body during the slide step should have a constant angle as far as possible
- the level (height) on which the hip axis moves during the slide step should remain as equal as possible until delivery
- to support this flat ball track, the knee should also continue the forward movement after which the foot has completed the sliding movement.

You also might want to read this:
Don’t Underestimate the Flat Spot!
http://tamerbowling.com/dont-underes...the-flat-spot/

Excerpt:
"So it is important to remember that the armswing should not be a simple semi-circle, i.e., it doesn’t just go back shoulder height and come forward shoulder height. At some point when you release the ball at the bottom of the swing, there actually has to be a slight forward projection which creates this “flat spot” effect."