In an effort to better understand the 'leg extension' rule that came into play last night, I sent an email to the Officiating office looking for clarification.  I wasn't looking for their opinion on the play last night  but just more information on the rule itself. Is it an immediate foul when an offensive player initiates contact by extending his leg?  Can it still be a defensive foul if the defender goes through the leg or is not set? Here's the response I received with references from the Rulebook and the Officiating casebook. 

...remember the referee has to make a judgement on what happened but
these are the rules:

From the "Comments on the Rules" Section in the rule book. (page 55)
Part C -- An offensive foul shall be assessed if the player initiates
contact in a non-basketball manner (leads with his foot, an unnatural
extended knee, etc.).

in our Case Book - Play 222
Defender B1 establishes a legal guarding position outside the restricted
area prior to Player A1's upward shooting motion. However, Player A1
extends his left leg in an unnatural movement and contacts Player B1 with
his foot first.
What is the ruling? An offensive foul shall be assessed against any player
who initiates contact in a non-basketball manner such as leading with a
foot or unnatural knee.