Plane, Steep Then Shallow, Over the Top

Plane
The easiest way to think of the swing plane is like this: If you reach down and touch your toes, your arms are hanging on a vertical plane (straight up and down). If you reach out and shake hands with someone, your arm is on a horizontal plane (parallel to the ground). A golfer needs to split the difference between these two extremes and swing on an angle that is between vertical and horizontal.
The degree of this angle you swing on is in large part determined by your height, arm length, what club you are hitting, and how far you stand from the ball. Hence, don’t worry about it too much. You will naturally swing more "vertically" (up and down) with a wedge because you are standing closer to the ball and have a shorter club; and you will swing more "horizontally" (level with the ground) with a driver because you are standing farther away and have a longer club.So plane, in a nutshell, is simply an angle you swing on. That is all you need to think about.

Steep, then Shallow
Okay, so now you know what plane is. Ideally, you would want to swing on that exact same plane throughout your entire swing. You would in essence have perfect swing plane. Unfortunately, because of the way a human is built, it is impossible to swing on one plane, or angle, in the golf swing. You need to swing on more than one plane; that is, you need to have separate backswing and downswing planes.

There is more than one plane in the golf swing. For 99% of the touring professionals, their backswing plane is more vertical than their downswing plane. And that is called swinging Steep, the Shallow.

You need to have a shallow (from the inside) downswing. And one of the easiest ways to achieve this, as is evidenced by the great players, is to have a backswing that is steeper. This will give you room to reroute the club to a shallow downswing plane.
Your swing can be. . .
Very steep and very shallow like Lee Trevino
Very steep and less shallow like Tiger Woods
Less steep and very shallow like Ben Hogan
It doesn’t really matter as long as it’s steep then shallow in some degree, and not...

Over the Top
To swing over the top means that instead of having this proper shallow downswing, you have a steep downswing. That is, instead or rerouting the club properly, you reroute it the other way, to a steeper plane. When you do this, your club approaches the ball from out to in, which encourages your clubface to open, which results in a slice.

Incidentally, one of the biggest culprits for this incorrect rerouting to a steep downswing is a shallow backswing. When you swing back shallow (inside/flat) your club has nowhere to go on the downswing but out and steep.
So, in effect, most over-the-top players swing "shallow then steep" instead of what tour players do, "steep then shallow."

To sum up PLANE, STEEP THEN SHALLOW, and OVER THE TOP. . .
When you set up to the ball you establish your swing plane. Problem is: You can’t swing on this singular plane. You instead want two planes in your swing: one for the backswing, one for the downswing.One of these planes has to be steep; the other has to be shallow. They don’t need to be drastically different, just different. The question now becomes when to swing on the steep plane and when to swing on the shallow plane. This will determine your golfing destiny.

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