Face guarding occurs when a defender blocks or disrupts the vision of the offensive player, whether that player has the basketball or not. If you place your hand in front of the eyes of a player in the act of shooting rather than defending the ball, you are guilty of face guarding. Similarly, when you are guarding a player away from the ball, if you block his vision to prevent him from receiving the pass, it is a violation.
The NBA does not specifically outlaw face guarding in theory, though if any contact is made while a defender is waving or placing the hand in front of the face of an opponent, a personal foul is called. Flagrant contact to the head or face often results in ejection and possible suspension for the offending player. NBA officials, as most fans know, are not in the habit of calling a foul when there is no contact to the face.
The NCAA defines face guarding as, “Purposely obstructing an opponent's vision by waving or placing hand(s) near his or her eyes” in Rule 10, Section 6, Article 1 in the official rules, which cover both men’s and women’s basketball. The offender is penalized with a technical foul resulting in a free throw for the opposing team.
The National Federation of State High School Associations first outlawed face guarding back in 1913, and it has left little room for interpretation of the rules since then. In 2004, the NFHS highlighted face guarding in its 2004 “Points of Emphasis” release for referees when the rule was modified to include actions occurring away from the ball. The release notes that placing a hand in the air while defending a player in the post is legal, but deliberately blocking the vision of the offensive player is not.
The International Basketball Federation’s rules follow closely to those of the NCAA and NFHS. Face guarding is explicitly prohibited by rule 38.3.1 which states, in part, “baiting an opponent or obstructing his vision by waving his hands near the eyes” is a technical foul against the offender resulting in a free throw opportunity for the opponent.