Clipping is a type of illegal block in which the offensive player blocks a defender from behind and below the waist. 
The penalty for clipping is fifteen yards and the down is repeated. Within three yards of the line of scrimmage, there are some versions of clipping that are legal such as blocks from behind that are above the knee and below the waist.
However, blocks below the knee are always illegal, nor can an offensive player roll into a defender from behind regardless of where they make contact. 
The offensive player approaches the defender in blue from behind, striking him below the waist.
- Clipping is called when a defender is blocked below the waist from behind
- Some types of clipping are allowed near the line of scrimmage, though hits below the knee are always illegal
- The penalty for clipping is fifteen yards. The down is repeated.
When is Clipping Legal?
Close to the line of scrimmage, clipping is allowed in a few instances. The NFL calls this area, “close-line play,” and is the space between the two offensive tackles and three yards on either side of the line of scrimmage. 
Within this area, clipping is allowed when the blocker makes contact above the knee. An exception is if the offensive player “rolls” into a defender’s legs after making a block. 
This is always illegal in order to reduce the risk of a player’s legs getting caught or tangled in a pile which commonly results in serious injury.
Why is Clipping Illegal in Most Circumstances?
Like other blocking penalties that invoke a fifteen-yard penalty, the safety of the defender is the primary reason why clipping isn’t allowed. One of the best examples is this brutal injury to former Texans star linebacker Brian Cushing.
Here we see Cushing blocked below the knee (which is illegal anywhere) and from behind. With the block coming from behind, the defender has no chance to avoid the block or protect themselves. The fragility of the leg can lead to devastating injuries, and unfortunately, Cushing tore his ACL on this uncalled clip, causing him to miss the rest of the season. 
A peel-back block is similar to clipping and also carries a 15-yard penalty and occurs more often in the open field.
A peal back block is considered illegal if:
1). The blocker is moving towards his own end line (the endzone behind the line of scrimmage)
2). He approaches the opponent from behind or from the side.
If the blocker can get both shoulders in front of the defender before throwing the block, it’s considered legal. 
The Patriots’ lineman is pointing back toward his own end line and makes contact from the side.
Blocking Below the Waist on Change of Possession
Blocking below the waist is not allowed during kicking plays. The one exception is between the tight-end box before the kick occurs. 
This is another rule that’s in place with the player’s safety in mind. Players are able to get up to top speed on these plays, and suddenly getting hit below the waist is a recipe for serious knee or leg injuries.
The low block rule also applies during other changes of possession such as a turnover or fumble recovery since these too involve players moving at top speed through space.
An Eagles defender performs an illegal block below the waist after a Cowboys’ interception.
Read more: WHAT IS FUMBLES IN AMERICAN FOOTBALL?
Clipping vs. Illegal Block in the Back
An illegal block in the back is a similar penalty and is called if a defender is blocked from behind and above the waist. 
This carries less risk of injury to the defender than clipping, and the penalty is only ten yards instead of fifteen. 
Even though the player in white isn’t using his hands to block, the contact is coming from the back and is illegal.
Unlike clipping, this infraction isn’t allowed anywhere on the field but is allowed if a player is attempting to recover a fumble. 
The infraction also isn’t called if a defender turns away from the offensive player at the last moment in an attempt to force contact from behind.
Clipping vs. Chop Block
A chop block is called when a blocker makes contact with a defensive player below the thigh when he’s already engaged by another blocker. This can be called whether the contact is made from the front, side, or behind and is always illegal. 
Like clipping, a cut block carries a fifteen-yard penalty due to the higher risk of the defensive player being injured.