You have to teach it without teaching it. As soon as you teach the student to IR, it will become a forced motion and result in a far less effective whip. The same thing happens as soon as someone teaches a student to "get open" during the drive.All that to say... we probably do need to teach I/R.
What is necessary is the set the stage and let the body handle things from there. If you teach to pull the ball down the back of the arm circle and lead with the pinky into release, the student will naturally IR without the teacher ever mentioning turning the hand over. A cue of "throw the ball, don't toss it" helps.
For stubborn cases, the reliable "bat drill" will show the correct way the arm works, all without ever mentioning IR to the student.