The split jerk is performed from the rack, and if you know that you need to work on your overhead flexibility please arrive early and do so. Here is a great video with some useful stretches (starting at the 3 minute mark) to achieve a better overhead position. 15.3 is a wall ball workout. Since you’ll spend the majority of the 14 minutes on this movement, you need to strive for efficiency. Austin Malleolo of Crossfit Seminar Staff offers a few simple tips in THIS video.