If I understand the explanation it sounds like a tight wind-up motion. If he starts with his right foot on the rubber and his left behind it, he can then step forward with his non-pivot foot. Then he can turn his right foot and place it in front of the rubber and continue his wind-up. It is accepted practice for the pitcher to lift his pivot foot and place it either on top or in front of the rubber. Although technically illegal, it has been determined not to call it.