There was a thread about this a couple of years ago.
new rule question
Mick and Jurassic Referee had the citations:
Originally posted by mick
OBSTRUCTING OPPONENT'S VISION
"10.3.7 Situation A: Does holding a hand or hands in front of the face of a player who has the ball, by an opponent who is in a legal guarding position, constitute unsporting tactics? Ruling: Yes. The described action is illegal. It is unsporting for a guard to take a position behind a post player, or to take a postion facing an opponent, or to take a postion with his/her back to the ball and facing the opponent and then in either case, wave or hold hands in front of the opponent's eye's so that the opponent cannot see. Holding or waving hands near the eye for the ostensible purpose of obstructing an opponent's vision is unsporting. (10-3-7; 10-6-1)"
Originally posted by JR:
And furthermore.......
POE 4A- Face Guarding- "Face guarding could occur with a single hand and a player's hand(s) do NOT have to be waving; the hand(s) could be stationary and still restrict the opponent's vision.The committee does not intend for good defense to be penalized. Challenging a shooter with a 'hand in the face' or fronting a post player with a hand in the air to prevent a post pass are examples of acceptable actions. The rule and point of emphasis is designed to penalize actions that are clearly not related to playing the game of basketball properly and that intentionally restricts vision"