blocking below the waist NF vs NFL

[Dommer1]

NFL Crackback rule:

If an offensive player is lined up more than two yards outside the tackle ("flexed player"), or if a back DURING THE PLAY moves more than two yards outse where the tackle lined up, he may not block below the waist if the blocker is moving towards the position of the ball at the snap. The block is illegal only of the block occurs within five yards on either side of the line of scrimmage.

However, if there is a scramble that clearly changes the flow of the play, you may legally crackback!