Take a look at these P5 (left arm horizontal, downswing) positions. The left is bad, the right is good. I've subtly marked this with RED for bad and GREEN for good. Yeah, I'm feeling a bit sarcastic today, too. ;-)
Anyway, if we had to give these two elbow positions TGM terms, the left would be more of a punch elbow position and the right would be more of a pitch elbow position. I'm striving for something more like what you see on the right, but it's not quite all the way to pitch, so I call it "pinch" elbow.
Now, the swing on the left is the result of a few things (backswing gets a little long, clubhead goes from slightly under plane and tips out to go slightly over the plane), but the focus for me is on the elbow position.
First, a detailed list of the differences.
- On the left, note the relative height of the right elbow as shown by the two-headed arrow. On the right, this gap between the right elbow and the belt is almost non-existent.
- Note the elbow positions relative to each other. On the left, the right elbow is still higher than the left elbow. On the right, the right elbow is lower than the left elbow.
- Note the relative height of the right shoulder. This makes sense, of course, since our upper arm isn't exactly going to expand or contract - so if the right elbow is higher, the right shoulder will stay higher.
- Note the angle of the right upper arm - over 25° difference.
- Most importantly, note the change in shaft plane. You've heard of keeping your elbow up in a baseball swing? Well, the dumbass on the left looks a lot more like he's playing baseball than golf.
Make no mistake about it - I've been able to play really good golf from the position on the left, just as I could play good golf from some so-ugly-you'll-vomit positions from a few years ago - but the motion on the right is much better.
Bonus question: what's the biggest (most important) difference I didn't mention in the list above? I'll tell you at the end. Think about it.
Now, there are a bunch of ways to feel this or fix this or get into this position. And, bear in mind, not everyone needs to get their right elbow lower or go "pitchier" in their motion. But if you do, try some of these feelings out to see if you can change the picture for the better:
- Obviously, fixing your backswing is going to help. Though it was terrible advice for Ray Romano, Hank Haney made a big deal out of "loop it the other way." Think of Tiger Woods - steep backswing, shallow the club through the transition. Rickie Fowler, too. If you're coming in from under the plane and the club tips out, good luck - sometimes fixing the backswing will allow you to make a better downswing. Jim Furyk swing (I've previously employed this feeling too).
- Squeeze the elbows together. In the left image, my right elbow is getting behind my body, but if I actively squeeze them together during the transition and first stages of the downswing, my right elbow will get more in front of my body (i.e. where my left elbow is).
- Try to get your right elbow to your belly button in transition. You won't get anywhere close, but get it as close as you can.
- Employ more right forearm fanning from P2.5 or so onward and don't let your right elbow get behind (or near) your shirt seam to begin with.
- Point your watch (on the back of your left wrist) to the sky.
- My favorite, because it works pretty well for me (came up with it on the golf course while practicing #2), is to feel like my right shoulder gets as far away from my right ear as possible starting at P3.8. Firmer wrists help - if the clubhead is flopping around it'll tend to tip out a little as the swing gets a bit sloppy-long. This works because, again, your upper arm doesn't collapse or expand, so if you drive the right shoulder down the elbow will go down as well.
There are just a few feelings. I can't give them all away! 
I hope this information helps you. I look forward to the discussion around this topic.
Answer to Bonus Question (Click to show)
Here's the answer to the bonus question:
The left arm is a few inches (or degrees) further "in" - it's still riding down the plane rather than being slightly on top of it, which will of course require compensations to hit the ball relatively squarely.


























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