Jordan Spieth has been woking with a swing coach expert in 2021. I won’t name the coach but let’s just say he created Tiger Woods’ greatest golf swing while he was holding the Tiger Slam. One thing that this particular golf swing coach teaches is a rotational and core golf swing; not an armsy golf swing. What amateurs do not realize is the only way to not have an armsy swing is to have soft arms. The left elbow actually falls inside towards the belly button throughout the backswing. When really perfecting this swing most people are going to need a soft left elbow at the top of the swing.
As the swing gets more engrained and there is time to increase flexibility a golfer can then work on straightening that left arm. If you looked at the physics and muscle movement of any PGA Tour Pro golfer they actually have their elbows in (towards their belly button) throughout the entire golf swing. Ben Hogan talked about feeling like the forearms were wrapped together; this is the feel. That said, no amateur does this because it feels weak.
The only way to actually swing the club with soft arms, and the elbows pointing in, is to have amazing core strength and great flexibility. It could take three to five years for an amateur with an armsy swing to create the core strength and flexibility to feel soft elbows and arms throughout the entire swing.
As Jordan Spieth continues to work on his swing, you will likely see that left arm get straighter, but for now he is working more on the rotation of the body and the flexibility of the left arm will come later. A deeper backswing with the core will allow for many flaws and faults when it comes to the hands and arms. Pro golfers are always working on a deeper shoulder turn with their body.