What is an illegal shift in football?

Answer 1

In HS: On offense, there must be seven or more players lined up on the line of scrimmage when the ball is snapped. If there are less than seven offensive players on the line of scrimmage at the time the ball is snapped the penalty is called an illegal formation. (if there are more than 7, some players that would normally be eligible to be downfield for a pass are ineligible, but it is still a legal formation).

In NCAA/NFL: There must be 4 players in the backfield (off the line of scrimmage) at the snap.

While the two rules sound the same, the difference occurs when the offense doesn't have all 11 players on the field. In high school, it means less players in the backfield. In the upper levels, it means less players on the line of scrimmage (and it makes the job of the short-wing officials a lot easier when checking the formation).

There are also some other rare/unusual rules when governing legality of formations, such as HS requires a man on the line of scrimmage on both sides of the center (usually the guard), in college and pros you need two players on both sides of the center on the line of scrimmage (usually the guard and tackle). Also, the 5 players on the line that are ineligible (the offensive lineman) must wear numbers of ineligible receivers (between 50-69). There is an exception to the player numbers on 4th down FG attempts and PATs.

Answer 2

This is when an offensive player moves when he is not supposed to.

Answer 3

Definition: A foul by the offense where a player fails to reset for at least one second after two players shift position at the same time.

Answer 4

It is called clipping. An illegal chop by a lineman under the knees.