A stiff arm, also called a stiff arm fend, is a type of ball carrier move in football where the ball carrier pushes defenders away with his hand that is not carrying the ball. This motion keeps the ball tucked inside a player's arm and also keeps a distance between the ball and the defender. The stiff arm fend always keeps the opponent at an arm's distance away at all times.
If the player stiff arms a player in the face or neck, a flag will be thrown, signalling a penalty violation. The ball carrier has to make sure he stiff arms the player's body. Restraining a player's face could result in a face mask call.
The stiff arm is considered effective because there is a lot of force can come through the length of an extended straight arm- enough to drive an opponent away from you.
Fun fact: Did you know that the straight arm motion is similar to what a martial artist would use to fend off an opponent?
The stiff arm fend can also be dangerous. It involves fully extending your arm, which can be injured if another player is running full force at you. The arm could be injured by being hyperextended or twisted upon impact. To ensure an injury doesn't happen, make sure the force is applied through a strong, intentionally straight arm. Stiff arm with intent and power!