Here is a post and video from two years ago…and the principles are still true. The vertical shoulder turn on the downswing (I have found) is one of the leading causes of early extension (Goat Humping)
Golf is action/reaction. Most golfers are too flat going back and too vertical coming through. Even PGA Tour players have these issues. Watch Dustin Johnson. He is so vertical coming into impact, he has to massively wrench the club flat and around him to keep the club from getting away from him. He does this with a hip turn that 1 in 1,000,000 people are capable of.
That’s fine for a World class player who manages a severe body action, but for most golfers, having the shoulders turn 90* to the spine on the backswing and downswing is the easiest way.
Most amateurs whip the club too far inside to start the backswing. This initiates a very flat shoulder turn and lots of late arm lift. This can result in rerouting over the top and a shoulder turn that is also too flat through impact…or more often, a shoulder turn that is way too vertical in order to “help the ball in the air,” or an attempt to come from the inside.
All of the techno babble aside, maintaining the T (LINK) or a 90* shoulder turn keeps things simple.
Here is a video describing some of the shoulder turn issues that cause problems.