Below Waist
ARTICLE 2. a. A block below the waist is a block in which the force of the
initial contact is below the waist of an opponent who has one or both feet on
the ground. When in question, the contact is below the waist (Rule 9-1-6).
b. A blocker who makes contact above the waist and then slides below the
waist has not blocked below the waist. If the blocker first contacts the
opposing player’s hands at the waist or above, it is a legal “above the waist’’
block (Rule 9-1-6).
ARTICLE 6. a. Team A prior to a change of team possession:.
1. While the ball is inside the tackle box, the following players may
legally block below the waist inside the tackle box until they leave the
tackle box: linemen completely inside the tackle box, and stationary
backs at least partially inside the tackle box and at least partially
inside the frame of the body of the second lineman from the snapper.
(A.R. 9-1-6-V)
2. While the ball is inside the tackle box, all other players (i.e., those
not covered in paragraph 1), and all players after the ball has left the
tackle box, are allowed to block below the waist only if the force of
the initial contact is directed from the front. “Directed from the
front” means within the clock-face region between “10 o’clock and 2
o’clock” forward of the player being blocked. Exceptions are given in
paragraphs 3 and 4. (A.R. 9-1-6-I-II, IV, VII-VIII)
3. Players not covered in paragraph 1 (above) may not block below the waist
toward the original position of the ball at the snap until the ball carrier is
clearly beyond the neutral zone.
4. Once the ball has left the tackle box a player may not block below the
waist toward his own end line. (A.R. 9-1-6-III)
Bolded part of 1. Above I believe was why it was called. If he'd engaged first then dropped down then it's a legal block, per the first two items.
Sent by my phone on a bullet train from Hillsboro.