I am trying to understand what a defensive foul is. What is the defender allowed to do? Can they not block the offensive player's path with their body? Looking at these videos it seems that the only thing the defensive player can do is to stand in the way of the offensive player without moving? Or can they move only away from the offensive player?
Any good videos showing what is allowed/disallowed are also appreciated.
A personal foul by a defensive player is covered in Section I of Rule 12b of the NBA rules:
Contact initiated by the defensive player guarding a player with the ball is not legal. This contact includes, but is not limited to, forearm, hands, or body check.
However, there are a list of exceptions that accompany this rule, which answers what a defender is allowed to do.
EXCEPTIONS:
(1) A defender may apply contact with a forearm to an offensive player with the ball who has his back to the basket below the free throw line extended outside the Lower Defensive Box.
(2) A defender may apply contact with a forearm and/or one hand with a bent elbow to an offensive player in a post-up position with the ball in the Lower Defensive Box.
(3) A defender may apply contact with a forearm to an offensive player with the ball at any time in the Lower Defensive Box. The forearm in the above exceptions is solely for the purpose of maintaining a defensive position.
(4) A defender may position his leg between the legs of an offensive player in a post-up position in the Lower Defensive Box for the purpose of maintaining defensive position. If his foot leaves the floor in an attempt to dislodge his opponent, it is a foul immediately.
(5) Incidental contact with the hand against an offensive player shall be ignored if it does not affect the player’s speed, quickness, balance and/or rhythm
Comment 2C on the rules also addresses what a defender is allowed to do:
BLOCK-CHARGE
A defensive player is permitted to establish a legal guarding position in the path of a dribbler regardless of his speed and distance.
A defensive player is not permitted to move into the path of an offensive player once he has started his upward motion to attempt a field goal or pass.
A defensive player must allow a moving player the opportunity to avoid contact when the offensive player receives a pass outside the lower defensive box. The lower defensive box is the area between the 3-foot posted-up marks, the bottom tip of the circle and the endline.
A defensive player must allow an airborne player the opportunity to land and then avoid contact when the offensive player is outside the lower defensive box.
A defensive player is permitted to establish a legal guarding position in the path of an offensive player who receives a pass inside the lower defensive box regardless of his speed and distance.
A defensive player must allow an airborne player who receives a pass the space to land when the offensive player is inside the lower defensive box.
