I have seen girls that have had this mechanics problem before, but none that I have worked with. It seemed to me at the time, that there whole motion, seemed to pause, when their arm was at the top of their rotation. That little hesitation might be caused by the pitcher trying to get a little extra on the pitch, and muscling the arm down the backside of the arm rotation.
Not sure of any specific drills to stop it, but you might want to try double, and triple arm rotations then releasing the ball. Trying to keep relaxed in her arm movements, but trying to gain arm speed on each rotation, before releasing the ball. I'm sure someone else may have some better ideas. Good Luck.