Which block is considered illegal if initiated on a player in motion?

A clip is considered illegal when initiated on a player in motion because it occurs when a player blocks an opponent from behind and below the waist. This rule is in place to protect players from dangerous hits that can lead to injury, especially when they are not in a position to see the block coming, which is often the case when a player is moving.

A clip specifically needs to involve a player who is in motion, and thus it's particularly important to understand that a prohibited action against such players is heavily penalized to ensure safety on the field. This rule upholds the sport's commitment to player protection, as blocking from behind can lead to significant injuries. The other types of blocks mentioned have different criteria and contexts where they may or may not be deemed illegal, but the clip's definition directly ties it to the condition of the player being in motion.