Blocking below the waist is illegal by players of either team after a change of possession, by the kicking or receiving team after a free kick, safety kick, fair catch kick, punt, FG attempt, or try kick. (12-2-5)
It's also illegal to block below the waist at any time against an eligible receiver who is aligned more than 2 yards outside of the tackle or if within two yards of the tackle is blocked below the waist after he goes beyond the line of scrimmage. (Penalty is called an illegal cut block-15 yard penalty) 8-4-5
Finally, the definition of a block below the waist is according to 3-3 is
when the initial contact is below the waist with any part of the blocker’s body against an opponent, other than the runner, who has one or both feet on the ground. A blocker who makes contact above the waist and then slides below the waist has not blocked below the waist.
Note: If an opponent uses his hands to ward off a block, and the blocker contacts the opponent below the waist, it is not a block below the waist unless the blocker is obviously intending to deliver a low block
