here is the rule and rationale for it as well as the rationale for not having it marked on the floor
New Rule. A secondary defender may not establish initial legal guarding position under the basket when playing a player who is in control of the ball (i.e., dribbling or shooting) or who has released the ball for a pass or try for goal. A secondary defender is a teammate who has helped a primary defender who has been beaten by an opponent because he failed to establish or maintain a guarding position. “Under the basket” is defined as from the front and side of the ring to the front of the backboard. A player is considered under the basket when any part of either foot is in this area. (Note: An approved ruling will indicate that a player straddling this area is considered to be under the basket.)
In establishing position in any outnumbering fast break situation, a player may not establish initial legal guarding position under the basket since there is no primary defender.
In both cases, when illegal contact occurs, such contact shall be called a blocking foul, unless the contact is intentional or flagrant.
Rationale:The committee is responding to overwhelming concern from the membership regarding contact under the basket. After some consideration of a restricted arc, the committee decided that court markings were not needed, but changed its rule in reference to secondary player establishing initial guarding position under the basket.