This is an ancient response (2012) and so the rules could have changed some, but it highlights the common understanding of linemen being set for one full second prior to snap.
"Those movements are treated the same as a shift prior to the play or a tight end going into motion. From page 98 of the college football rule book (
http://www.ncaapublications.com/productdownloads/FR09.pdf) (emphasis mine):
ARTICLE 4.
a. If a snap is preceded by a huddle or shift, all players of the offensive team must come to an absolute stop and remain stationary in their positions, without movement of the feet, body, head or arms, for at least one full second before the ball is snapped (A.R. 7-1-4-I) [S20].
b. It is not intended that Rule 7-1-4-a should prohibit smooth, rhythmical shifts if properly executed. A smooth cadence shift or unhurried motion is not an infraction. However, it is the responsibility of an offensive player who moves before the snap to do so in a manner that in no way simulates the beginning of a play. After the ball is ready for play and all players are in scrimmage formation, no offensive player shall make a quick, jerky movement before the snap, including but not limited to (A.R. 7-1-4-II-IV):
A lineman moving his foot, shoulder, arm, body or head in a quick, jerky motion in any direction [S19]."
The comment on tight end being in motion implies legal receiver so not covered. Back and sideways motion by backs and receivers is allowed but not towards LOS. You often see shifts re-aligning 7 on line when a receiver goes in motion.
The number thing is to allow the defense to recognize those who can and cannot receive. Anyone can block, though, but the rules outside the central line has more restrictions in blocking below the waist:
"Linemen with initial position completely inside the tackle box may legally block below the waist inside the tackle box until the ball leaves the tackle box. All other Team A players are allowed to block below the waist only if the force of the initial contact is directed from the front. "Directed from the front" is defined as within the clock face region between "10 o'clock and 2 o'clock" forward of the area of concentration of the player being blocked.
Exceptions:
-Team A players may not block below the waist when the block occurs five yards or more beyond the neutral zone.
-Players outside the tackle box at the snap ,or any time after the snap, or in motion at the snap may not block below the waist toward the original position of the ball at the snap.
-Once the ball has left the tackle box, a player may not block below the waist toward his own end line."
It's a complex topic. And note that cut blocks are not chop blocks but still can be legal or illegal by the rules.
What used to get called as clipping is now block int back:
https://en.m.wikipedia.org/wiki/Clipping_(gridiron_football)This isn't all directly relevant to the original ask, it it helps frame several contexts for the specific rules. Note tight ends are outside the tackle box UNLESS between the tackles and therefore ineligible as a receiver. So they cannot cut block…assuming it's called of course…
