Old thread, but worth revisiting this issue as the rule has changed yet again. The "low blocking zone" has been thrown out, everything now referencing the tackle box. Here's the skinny:
Lineman inside the tacklebox at the snap, as well as stationary backs at least partially in the tackle box at the snap, can block below the waist from the front and the side as long as the blocker and the ball are still inside the tackle box. Once the ball or blocker leaves the tackle box, all blocks below the waist must be from the front (10 to 2 on a clock face).
Players outside the tackle box at the snap cannot block below the waist from the side on the play at all. Additionally, players outside the tackle box at the snap cannot block below the waist back toward the original position of the ball (crackback block) even if it is 10-2. They can go high coming back inside, but they cannot go low. This prohibition ends when the ball cross the neutral zone.
One final rule -- once the ball leaves the tackle box, no player may block below the waist facing his own end line (peelback block).
