even if some are clear on it, many are not... so even if we clear everything up here, that doesn't help... but...
1) as JasonTX alluded to, it is illegal to block below the waist in any shape, form, or fashion on any kick play or after any change of possession. Period, the end.
2) the tackle box is defined as extending 5 yards from the snapper in each direction along the neutral zone and running parallel to the sidelines. that box extends from the neutral zone to the offenses end line (through the end zone) the tackle box goes away as soon as the ball leaves that boundry.
3) a player who is stationary in this imaginary box at the snap is UNRESTRICTED, and beyond the neutral zone can block below the waist in a North, East, or West direction, he may not block at any degree to the south while BEYOND the neutral zone. These unrestricted players can block below the waist in ANY direction when they are behind the neutral zone.
4) a player who is stationary outside this imaginary box is deemed RESTRICTED, and beyond the neutral zone can only block below the waist in a North direction or towards his adjacent sideline. Behind the neutral zone, he may block North or South, but still only towards his adjacent sideline.
5) players in motion, this is where it gets even more difficult. If the player was restricted when he started his motion he remains restricted even if he is in the tackle box when the ball is snapped, if he is at or past the snapper while in motion, his adjacent sideline is determined to be the sideline towards which he is moving. If a player was unrestricted when he started his motion, as long as his motion did not take him outside of the tackle box, he remains unrestricted. If his motion takes him outside of the tackle box, he becomes restricted and has the same guidelines as other restricted players. If a player is restricted, either because he started in motion from a restricted position, or because he left the tackle box, he can establish himself in the tackle box and become unrestricted. That is, he must get set (come to a complete stop) for at least a second inside the tackle box.
6) any RESTRICTED player is prohibited from blocking below the waist inside the tackle box in any direction.
I am tired of typing... I think that about gets it all.
OH... defensive players may ONLY block below the waist in a 10 yard belt that extends for 5 yards beyond and behind the neutral zone from sideline to sideline, and they cannot block below the waist in this zone against players who are in a position to receive a backwards pass, and they can't block an eligible receiver beyond the NZ unless attempting to get to the ball or ball carrier.