[JRutledge]

Here is the rule in the rulebook.

4-61

Section 61. Secondary Defender

Art. 1. A secondary defender is a teammate who has helped a primary defender after that player has been beaten by an opponent because he/she failed to establish or maintain a guarding position. A defensive player is beaten when the offensive player’s head and shoulders get past the defender.

Art. 2. A secondary defender is a teammate who double teams a low post player.

Art. 3. After an offensive rebound, there are no secondary defenders when the
rebounder makes an immediate move to the basket.

Art. 4. In an outnumbering fast-break situation, any defensive player(s) initially shall be a secondary defender.

Peace