I play with a guy who is really trying to improve his swing. He lacks driver distance and has inconsistent iron play...chunks, tops, low & thin shots. So I shot a video of his swing to discover what is causing the problems, mainly reducing it down to a root cause analysis if you will, because all too often there's one or two main defects that create a bad chain of events.
At address he looks good. Good grip, weight centered on his feet, not too bent over or upright. Check.
Then as he takes the club back to about the halfway point, his shoulders start to flatten, or his left shoulder moves up and away from the ball. As the flattening movement unfolds, his trail elbow moves lower, more behind his torso. At the top his shoulders are nearly parallel to the ground and his trail elbow is barely visible in the video.
Given his shoulders are flat at the top, the club will naturally want to travel on a more outward path, but it can't because he'd shank or whiff at the ball, thus his brain realizes he needs to get the club traveling more downward. So his lower body tries to make an initial lateral move...but his lower body abruptly stops. His lower body stops because in order to make room for his trail elbow to come back around, he needs to post-up off his lead leg early to create room for the club handle. The early post up also acts as a speed brake (early extension), thus his swing speed maxes out halfway down, and moves his swing arc to his trail side. The interesting part is he spent a lot of time trying to initiate his downswing with a lead knee left move but it was never going to happen because that move created more disconnect due to the buried trail elbow. The age-old working on a "fix" that isn't the root cause of the problem. The interesting part is how one movement, or the flattening of his shoulders on the takeaway, leads to a series of other movements, and unfortunately, swing killing movements to hit the ball.
So at this point he is working two things. A left shoulder down...DOWN...takeaway, and equally important, extension in his backswing. The other is what's called the "pump movement" which has been around since Harvey Penick discovered the movement / drill. No hitting balls, using mirrors and video. Practicing shoulder down, extension, then pump movement...slowly...over and over. The left shoulder down along with extension positions his trail elbow in a more elevated, "roomier" position at the top. The pump movement thereafter reveals how fast his hands (or trail elbow) need to drop (not pulled down) to the trail knee area when combined with a lower body initiated downswing. In my experience, my lead knee posts up, or extends, at about the exact same time which makes that move an independent variable for which then the dropping of the trail elbow is a dependent variable to the post up. The two have to be timed to work together. I think pro's also struggle with this because even a slight miss-timed swing can lead to missed shots. I'm looking for 70% success, and even the pro's aren't robots. But even a slightly miss-timed swing produces far better results.
As for the pump drill, in S&T we're more on our left side, and with Hogan Hips, our lead hip is prepositioned to complete a lower body initiated downswing. As such, our lower body has a "head start" such that our hands / trail elbow need to drop pretty darn fast to match or take advantage of our prepositioned lower body movement. The pump movement is great (IMO) because one of the main issues with golfers is they cannot for various reasons such as those outlined above, get their hands moving down fast enough in the downswing, which leads to flips, chicken wings, tops, loss of power, thin shots, etc.
https://www.youtube.com/watch?v=9jy8W83MSWw
@GolfLivesMatter complicated issue.