I've drawn lines between the end of the shaft, the handle of the shaft and the right elbow. These points create a plane. This is the plane on which the club is swung.
Notice Mi-hyang doesn't lose this plane until after impact. Most people lose this plane way too early in the downswing. Think of this plane as a paper kite. If you move it on it's plane it will stay intact. If you move it against its plane, the paper kite will rip.
This plane is dependent on the position of the right elbow. It acts as a pivot point and fulcrum that the right arm and club cantilever around.
Also take note of how much she's facing away from target as she starts the downswing and how much her head is turned away. This top of the backswing, completely turned away from the target position is where we hit the ball from.
That is because this is where we start the downswing, we have to imagine that we're hitting the ball from this pose. Most people try to return to facing the ball and then swing, so they're trying to turn and swing at the same time.
This is not a natural move.
When we imagine striking the ball from the top of the backswing position, we have the sensation of hitting the ball backward. I say "sensation," because that's all it is.
When you do this, your body actually turns naturally through the shot just as Mi-hyang's does, but it happens without conscious effort from the player.
If you think about it, in baseball we set up with the bat up, back and cocked - ready to swing and we hit the ball from that position.
We don't "try" to turn our body toward the target and then swing, we just swing with the sensation that our back is facing the pitcher because when we began swinging, it was. In golf unfortunately we need to measure our distance to the ball before we can strike it with any confidence, we call that position "address." Golf would be much easier if we didn't have to address the ball.
People believe this is the position where you strike the ball, and that's just completely wrong. In golf, just as in baseball, we need to imagine hitting the ball from the top of the backswing while our back is facing the pitcher/target.
This allows you to swing the club naturally, retain lag and release the club in a relaxed motion. It also gives us the sensation that our body is moving less and therefore we can swing more smoothly and confidently.
This is what gives those players that "effortless look."
Good drill to learn the golf plane
Let's start with the simple things. Swing your left arm in big circles, similar to a golf swing. Notice that it does not move or rotate perpendicular or parallel to your shoulders or spine.
Keep swinging your arm and turn your shoulders and torso so that the circle your arm is making is parallel to an imaginary target line.
Notice that in order to get your arm to swing naturally in the direction we want it to, your shoulders have to be tipped and tilted about 45º backward, or away from the target.
This is the secret to golf. Most people attempt to turn their shoulders back to square before striking the ball which causes the left arm to move down and left, or over the top.
Although, you think this might be odd, it's not. Everyone sees the pros finish facing the target and falsely believe we should be half way there at impact, if not more.
Once you learn to do this you'll feel that sensation that the great ball strikers have of hitting the ball with your head back and stable and then turning through to a finish that faces you toward the target.
You should have the sensation that you're hitting the ball slightly backward from the direction you're facing.
Try that first. Let me know how that works for you and we'll go from there. Yeah? That may be all you need.
BTW, this should feel quite comfortable and easy. If the ball goes wildly left or right your grip is not correct. Alter the grip until the ball flies straight without changing the swing itself at all.
One more thing, you have a natural swing speed.
If you try to swing the club much much faster you'll disrupt your body's natural motion and rhythm and the ball will do all sorts of strange things.
Swing comfortably but powerfully and the ball will go far and straight