What's the exact rule on the arm moving forward rule?

[prairiedawg]

looked like McElroy was pulling the ball back down and lost it, the refs said his arm was moving forward so it's an incomplete pass. I guess no matter what the qb does if the arm is moving forward it's a pass?

Edited to add: I know this is bama so it wouldn't matter if he was carrying it at his side the zebras would still say incomplete pass so it's more of a general question.

 

[maroondawg]

was he was trying to stop the throw and his hand/ball hit the OL in front of him and thats when the ball came out. But it could have came out just before that.