Here's a couple of very good videos from Athletic Motion Golf:
What I like about the videos is that they address key components of the transition phase which I believe is the most important part of the golf swing. I feel the transition really dictates the downswing mechanics as a whole and in particular what your impact conditions are going to look like.
I believe that a reason why golf is so difficult is because of the transition motion is difficult to master and that is due to the sudden change of direction and speed that is occurring at. If I put in enough range practice, I could 'master' any backswing change I wanted to make. The speed of the backswing is slower and speed plays a large role in how quickly you can learn new movement patterns. Not only is the downswing faster, but it mostly starts with the transition move and that area has speed and a change of direction and that's why golfers struggle and good golfers can struggle to make changes to their downswing.
In the first video, I like how they address 'weight shift.' In particular WHEN it needs to happen (prior to P5). Here's a video of Victor Rodriquez on Swing Catalyst to help illustrate that point:
Victor uses what is called a 'fish hook' CoP trace. This is quite common for the super long golfers like Victor where their Center of Pressure trace goes towards their lead foot in transition and then as they continue to rotate the body into impact, the CoP traces back towards the middle of his stance.
The second video destroys the idea of 'wide to narrow to wide.' This is something I never quite understood as when I worked with M.O.R.A.D. one of the things that was preached was to straighten the right arm out in the downswing and not pull the hands into the body. You could clearly see the best ballstrikers doing that, but you couldn't understand why their hand path got narrower.
The only thing about the 2nd video that needs to be re-emphasized is that there is a blended motion the shoulders make in the downswing. One is that they will tilt. This comes from the arms pulling down. If you're at P4, the rear shoulder will be higher than the lead shoulder. As the arms pull down, the shoulders will tilt to the point where the rear shoulder is lower than the lead shoulder.
The other motion that good ballstrikers make with the shoulders is they rotate them (thru the rotation of their upper torso). See how Dustin Johnson's shoulders are open at impact, below.
The reason why many teachers like George Gankas are now preaching to 'leave your hands up' and 'don't pull down' is because the vast majority of golfers always get the shoulder tilting motion and do not get enough of the shoulder/chest rotation motion. There is little in the way of blending the shoulder tilt and the shoulder rotation and it's mostly all tilt and little rotation.
So these instructors work on exaggerating the rotation and trying to FEEL like the golfer is not pulling down the arms in order to not tilt the shoulders too quickly.
I tend to look at it like a race. My arms are going to pull down and tilt the shoulders eventually. My goal is to FEEL like my shoulder/chest rotation is beating my shoulder tilt. In reality, when I'm swinging well it's almost like a synchronized blend of both motions.