You don't want to consciously think of drawing an "S curve", which can cause overuse injuries shoulder and rotator. You only need to think the arm is coming straight back. A natural "S curve" will happen as a consequence of correct position and timing as you roll from one edge to the other. Coach Dave describes this best in this video: High Elbow Catch
Here's Shinji in Before After Ti video. Watch from 00:24 the below surface front view in slo-mo. You will see the path of the underwater recovery as the body rotates from one edge to the other; as shoulders are narrow on one edge, widen at transition, then narrow on opposite edge - the natural "S curve" happens with body rotation.