What role does the left arm play in the backswing?

The position of the left arm in the backswing is a very important factor for a good golf swing. However, for many golfers it is also extremely difficult to control. For me, the left arm in the backswing is actually one of the most important control points for a safe and repeatable golf swing - especially when you are under pressure. Good physical conditions are required for this.

Take a look at the backswings of tour pros like Dustin Johnson or Rory McIlroy. It's impressive how these players manage to turn up so far and keep their left arm so straight at the same time. Try it yourself with a practice swing and see how far you can swing without your left arm bending. You will realise that this is not too far. The deficit is flexibility in the shoulder and arm area.

The tour pros train specifically on flexibility so that they can keep their left arm straight in the backswing. This is something you can work on specifically. But don't worry, you can also play good golf without a straight arm.

You will achieve the best effect if you stretch your left arm in the address position and try to keep it stretched as long as possible in the swing.
Your backswing should definitely have some shoulder rotation and not just consist of raising and lowering your arms. This is particularly noticeable when you have different impact moments (thin, fat).