This has come up as a point of dispute between a playing buddy of mine:

He asserts that as a defender, I cannot have my forearm in his low back when he has his back to the basket. He asserts that that technique can "only be used to maintain a defensive position", the definition of which he says is only maintaining my position between himself and the basket, if he is not dribbling or moving.

The way I always have heard it is that if the offensive player is in the post below the free throw line, I may have one forearm in their low back to check them. I think that I can use my forearm if he's below the FT line to keep him checked.

He ALSO says that when in the post, with me as the offensive player, I cannot shift my hips backward into him to move towards the basket while I have my back to it, as that is an "offensive foul like charging". I completely disagree, you see post players in the NBA all the time dribble with their back to the basket and using their weight to move the defense around.

Who's right?