What is football clipping? In short, clipping is the act of blocking or making contact with an opposing player, from behind, at waist level or below. It is considered a major penalty ( as opposed to a false start or illegal motion penalty, both of which are strictly technical and minor) because it not only gives the offender and his team an unfair advantage, it can cause serious and permanent injury to the victim.
The official calling the penalty will signal it as such by making a chopping motion to the back of his thigh. Clipping should not be confused with the penalty for a “block in the back,” where the offender also contacts the victim from behind, but at a point above the waist. Blocking an opponent in the back (but above waist level) is far less dangerous than clipping him, so the penalty is not as severe. At all levels, from high school to the NFL, a block in the back calls for a ten yard penalty, while a clip will bring a fifteen yard walk-off. Also, any player can be guilty of clipping: offense, defense or special teams. If the offending player is on defense, the penalty carries, not only fifteen yards, but also an automatic first down for the opponent.
It may come as a mystery to some why it is perfectly legal to bring down a ball-carrier from behind, while it is illegal to bring anyone else down by clipping. There is some logic to that concept, though. The ball carrier expects his opponents to bring him down. There are some ways of doing that, such as pulling him down by his face mask, that are clearly illegal, but, otherwise, being put on the ground, even from behind, comes with the territory.
Likewise, a defender can expect that the opponents will try to interfere with his efforts to reach the ball-carrier, by getting in his way with a legitimate block that he can see and react to. If a blocker is so powerful that he actually knocks the defender down, that is perfectly legitimate, as long as the block itself was legitimate. But, by not having possession of the football, the defender, or anyone else on the field, for that matter, has the right not to be blindsided, such as would happen with a clip. A well-executed clip can give its victim a lifetime of knee problems. It is considered a major penalty for a good reason.