but D', my point is - without a tool and visually seeing it happen, one can not assume the pitcher did it with his fingernail. Therefore, the reason it is so effective. The miracle is the ability of a pitcher to take advantage of such a small aerodynamic defect. From what I understand, it allows the knuckle ball pitcher to throw their pitchs faster.
As effective a tool is the fastball pitchers (Clemens, Ryan, Gibson, etc) who brush the batter off the plate - or, actually hit them. A momentary mental flinch at 100+ mph is very devastating