[Daryl]

Hold your Left Wrist with your Right Hand at the top of the Swing. Keep holding it during Startdown.
Lateral Shift to get the Right Shoulder Downplane forces the Left Shoulder Up-Plane. But if you don't let go of your Left Wrist, then your Left Arm will not come away from your chest. If you don't hold your left arm it comes away almost immediately.
Anti-Extensor action, you can hold the club up and delay letting it move away from the Right Shoulder.