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Originally Posted by DG

And, it is not a rule in FED, but an Interpretation, from 2005, that has not made it into the casebook. I have never seen it called.

It's in the 2012 Fed Casebook (and 2011): 6.3.1 (P)

6.1.3 SITUATION P: With a runner on first, Team A right-handed pitcher is in
the set position, bent at the waist and his pitching arm naturally hangs down
slightly in front or to the side away from his body. As he looks to the catcher for a signal, a) the pitcher’s arm is stationary or b) the pitching arm rocks slightly from side to side. RULING: In a), the position of the arm is natural and can be considered by his side in meeting the rule. Any movement would then start the pitch. In b), any movement of the arm is considered the start of the pitching motion and a pitch must be delivered to the plate so this motion results in a balk.