The "flat-spot" as I understand it is referring to the swing motion:

If you watch the motion described by the bowling fingers, with a more 80s style delivery you get this.

   |      |                         |           |
   |      |                         |           |
   |      |                         |           |
   |      |                         |           |
    \    /                           \         /
     \  /                             \       /
      --                               -------

The Flat spot, is the period during which the ball is travelling parallel to the lane prior to release. The longer this flat spot, the greater the leverage that can be generated. Usually the thumb comes out at the beginning of the flat spot, with the ball coming off the fingers at the end. The longer the flat spot, the longer the fingers are in the ball alone.
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Reporting from England

The flat spot comes before the release, at the bottom of the arm swing. The easiest way to create one is by keeping the torso more upright. If the bowler leans forward the ball comes downward much more steeply and the bowler 1) loses it off his hand too early, or 2) uses way too much muscle trying to avoid #1. Posture is the key.


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Lane Carter, Strike Zone Pro Shops - Salt Lake City, Utah
Brunswick Amateur Staff