When is a block not considered legal?

A block is considered illegal when initial contact is made from behind at or below the waist because this type of action can pose a significant risk of injury to the player being blocked. The rule is designed to protect players from potential harm that can result from below-the-waist blocks when they are not aware of the approach from the back. It emphasizes player safety, reducing the chance of serious injuries that can occur during violent impacts, especially in situations where a player may not see the blocker coming. This rule is critical in maintaining safety standards in the game while allowing players to engage in blocking techniques that follow the regulations.