Ideal timing is to release the ball (read: let the thumb go) just a moment before the forward swing with the ball passes your sliding foot's ankle, right under your body's CG.
It is the lowest point of the swing path, and ideally you propel the ball out onto the lane in a flat plane this way. The ball will sting "hang" on your finger tips, and this hold creates the revs as you "follow through".
The thumb should not hold onto the ball beyond that point, because this really kills the shot, makes you come around the ball and top it, and creates needless loft.
Fixing this timing can be tough - I know it from personal experience. Sometimes it helps just to let the ball consciously go, until you feel familiar with it.
You might also check whether you bend forward at the waist at the foul line - this also promotes late timing and problems with the thumb. Keeping the upper body more upright might help here, too.
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DizzyFugu - Reporting from Germany
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