I have a different approach. If I have a 12U pitcher with good, repeatable mechanics I will start to teach her about spinning the ball. I start by throwing her overhand curves and have her throw them back to me. I will then point out the spin on her fastball. We will then experiment with a palm up curve to see what kind of spin she generates and go from there. As much as everyone talks about pitchers with 6 pitches I have seen far too many 16U's who can't spin the ball and have no clue about the concept. I use marked balls in these workouts to make the spin more visible.