2014-15 Points of Emphasis- Officials manual

[bigjohn]

When the ball is snapped to a back in shotgun formation, however, the ball leaves the zone, and the zone
disintegrates almost immediately. To be legal, a block below the waist must occur immediately after, and nearly
simultaneously with, the snap. Any delay would cause the block to occur after the ball has left the zone. It is
nearly impossible for a lineman in a two-point stance to legally block below the waist in this situation because
of the time required for the lineman to drop from an upright position and block an opponent below the waist.
For linemen in three- or four-point stances, they must block their opponents immediately after the snap in order
for a low block to be legal in this situation.

https://www.nfhs.org/media/1015055/2...f-emphasis.pdf