Depends on the angle of release. The 6to 9 curve goes straight down. Not to mention the movement of a split finger fastball going down. This has been studied to death by scientists, but the fact remains that the ball does move. Some pitchers make it move differently than others
Of course it moves. But for it to actually "break" would require the ball to suddenly change its rotation somehow -- doesn't seem like that could happen on its own. It's probably more a matter of how we perceive the motion of the ball as it follows its (real) curve.