Ted Fort with a great explanation of 'swing plane.'
One of the great examples of a golfer who is beautifully on plane is one of Ted's friends, former professional Louis Brown.
There's a big desire and quest to have a 'single plane' or in TGM terms a 'zero shift.' Basically this is where the golfer tries to stay on the same plane throughout the golf swing.
There's talk that Moe Norman was on a single plane, but the actual video discussing this shows him on 2 planes (Turned shoulder and elbow plane). In Moe's case he was what they call in TGM terms a 'double shift' swing plane variation. He went from the elbow plane, to the TSP, then back to the elbow plane on the downswing.
I actually use a single shift which is still a '2-plane' swing. Remember, only a 'zero shift' is what they call a '1-plane' swing.
However, I tend to believe that the downswing in golfer's is too upright and they need to find ways to flatten it out. I'm not the biggest fan of staying on the TSP in the downswing...even in my own swing. I think if you have a flatter downswing you're likely to hit the ball more consistently and accurately because the way the clubs are built they really are not made to be swung upright...although I believe today's clubs tend to make golfers swing more upright than they ever should.
The big issue is if you naturally swing on the TSP on the downswing, you may want to avoid try to go to the elbow plane because it's often a futile attempt in the end. If I were going to suggest a way to do it, I would suggest finding an old forged 5-iron and bending 2* flat and hitting balls with it. Eventually your brain will tell your body that you need to flatten out your downswing. And as you get better, then bend the club a few more degrees flatter.
Since it's an old, beat up club if the club snaps from bending it then you are not out a new club or some decent amount of money. However, John Erickson...known for bending clubs...says he's never had 1 forged club snap on him when he bends them and says the most he ever bent a club was 12* flat!
Anyway, like Ted stated it really doesn't matter if you swing on a flat plane or an upright plane, but that you 'stay on plane.' However, I just find that if your *downswing* is more upright, your margin for error is less.
Also, if you look at most current Tour pros and most of the great golfers of the past, they almost all use the 'double shift' (elbow-TSP-elbow). Most of these golfers were not into the science of the game, but I believe they did this because their brains were in tune that in order to hit the ball as well as they possibly can and do it consistently, they needed to swing on a flatter *downswing* plane.
In fact, I really don't believe the 1 Swing Plane actually exists. And I've come across quite a few golfers that have tried it and to a man they all say they can't hit a driver when doing it. I know Tiger's coach, Hank Haney, worked under the '1 Plane Swing' founder Jim Hardy and is trying the 'parallel planes' that Ted talks about in the video and he also struggles with his driver.
Another key factor in plane is 'staying on plane' after impact. This post by friend of the 3Jack Blog, nyc lagster, does a great job of explaining why 'swinging left' is important.
As you can see, by 'swinging left' the golfer keeps the club on plane post impact.
I really think that's what teachers like SliceFixer, Geoff Jones, Mac O'Grady, Manzella and others are really doing with their instruction...trying to get the golfer to stay on plane after they hit the ball by 'swinging left.' How they teach 'staying on plane after impact' is a whole different thing.