The NFL’s rules restrict how offensive players may block their opponents; we have already learned about the holding penalty. Below is a list of additional violations and brief explanations:
1. Block in the Back: Under the rules of football, you may not block by pushing an opposing player from behind.
2. Clipping: A variation of a block in the back, clipping occurs when the offensive player dives below the waist of the defensive player from behind. Clipping above the knee is legal, however, in what are known as “close line plays.” These plays can occur in the space between the offensive tackles and three yards on either side of the line of scrimmage.
3. Chop Block: A chop block is when an offensive player blocks the defensive player below the knees. This will be a penalty if the defensive player is already engaged with another offensive player and the second offensive player joins the block, but below the knees. As you can imagine, players can get seriously injured with these blocks. It won’t usually be called, though, when the block occurs in the open field.
4. Tripping: It is illegal to for a player on offense or defense to use his foot or leg to trip an opponent. In addition to a ten-yard penalty for the team, a player can also face fines for this violation. See this article for an example.
5. Crackback: This is a block below the waist by a player who is an eligible receiver—i.e., not an offensive lineman. This occurs when the player started the play in a position two yards away from the offensive tackles, but then moved back inside (i.e., toward the spot of the ball at the snap) to block. A common example is on a reverse when the ball is handed to a player who then runs behind the line of scrimmage toward the other sideline. Here is an example of Minnesota Vikings quarterback Brett Favre executing a crackback during a different kind of play:
With the exception of a crackback block, the penalty for all of the above violations is ten yards from the spot of the foul, or ten yards from the previous spot if the penalty occurred behind the line of scrimmage. A crackback block is considered a personal foul and is therefore penalized with fifteen yards.