Train the right arm to extend the left on the backswing. That’s called “extension”. Use the “Toolbox” backswing set up to drill.
The Impact Zone® Toolbox is golf’s only ALL-IN-ONE training aid! The toolbox has over 100 different uses as a training for all aspects of the game, including as a way to measure low point and analyzing your divots, path, weighted swing trainer, stretching, plane, ball position, alignment, and as a camera stand. The components of the toolbox can be used for putting, chipping, pitching, and full swing.