Here is a link to NBA Post Up's on YouTube. Sorry about the music, just mute it.
YouTube - Post Moves
Starting at 1:03 with Carmelo who uses his Butt to create space you will see there was no call. This is the stuff I'm talking about.
Also Boozer at 1:40 is the prime example of what I am referring too. That is the move Shaq and other big men have done their whole careers.