Where many golfers go wrong is they stop turning their chest. Once the body stops rotating, the arms have nowhere to go, so they begin to bend instead.
This simple drill teaches your body that your chest turn supports your arm extension, helping you create a wider, more connected backswing.

Keep in mind, a bent lead arm isn't always a flaw. Your grip, flexibility, body type, and swing style all influence how much your arm bends. But if you're collapsing your backswing and losing width, this is a great place to begin.

Train the movement. Trust the motion.