Sitting in a chair and twisting to the left will cause the muscles around my back on my right side to gather up or shorten.
Mmmm... not necessarily. That will only happen if you use the wrong muscles. It's easy to think you're rotating the entire top when in fact you're disengaging the shoulders, causing distortion. When only the long muscles closest to the spine are used, the entire top rotates as a unit. The trick, of course, is finding these muscles, and this generally requires individual instruction.
One of the reasons I'm very specific about the position of having the fingertips touching the solar plexus with the elbows outstretched is because it tends to reduce the possibility of isolated shoulder usage. It doesn't make it impossible for the truly determinied, but the average student responds well to this position, and can use it to learn to find the right muscles so that eventually they can do it correctly without this "crutch".
Also, don't forget that the dance position itself is not symmetrical. So if you're trying to achieve a perfectly symmetrical position within your own body, the end result will be distortion.
Imagine three straws of equal length, connected by a rubber band running through them. Straw #1 represents the line from your left elbow point to your left shoulder, straw #2 represents the line across from left to right shoulder, and straw #3 represents the line from right shoulder to right elbow point. If you begin with all 3 straws forming a straight line across, with the outermost points fixed, then twist the center straw to one side or the other, it will result in a zig-zag shape. This is what happens to students in dance position. The zig zag shape is what gives the topline a broken appearance.
For ladies in closed dance position, typically it's the right shoulder that protrudes slightly forward -- or as some might prefer to describe it, the left side drops down and back (Ultimately, however, it's the same thing). This is not an accident. It's not that all females tend to be lopsided. The problem is that the dance position is offset, which means that the hands and elbows when in dance position are actually shifted to the right of the body. So when you're facing straight forward with your hands and elbows in this position, you're actually distorting your topline. The way to fix it? Rotate your sternum to the right. Now it's positioned directly between the elbow points.
If the dance position was such that we were standing directly in front of our partner, then it would be much easier to maintain a balanced topline. But the offset position necessitates some rotation of the torso in order to maintain the balance. Without it, your topline becomes distorted.
This is so much easier to demonstrate than to write about. But I hope what I've written makes sense.
Regards,
Jonathan