What is defined as a chop block?

A chop block is specifically defined as a type of illegal block that involves two players from the same team blocking an opponent simultaneously in a manner that targets the opponent's knees or lower body. In this case, one player is engaged with the opponent above the waist, while another player makes contact with the opponent at or below the knees. This rule is in place to protect players from serious injuries that can occur due to these kinds of blocks, as they put the player in a vulnerable position.

The other options describe different types of blocks that do not fit the specific criteria of a chop block. A block from behind is a separate violation related to contact initiated from the rear, while a block targeting the ball carrier involves engaging the player with the football, not another opponent. Finally, a block initiated after a player is down does not qualify as a legal block under any circumstances in football and is considered unnecessary roughness.