I'm sure this isn't a complete answer, but it's how I like to think of it:
It's a method for approaching things you do everyday, whether as a singer, instrumentalist, actor or computer programmer, by being mindful of your posture and where you hold tension. It's particularly helpful for performing artists because tension in the wrong places is not only a gateway to injury, but it also inhibits expression. For instance, after my AT teacher works with me on my posture, my tone is immediately and noticeably better on the flute. It's also easier to do everything when all the unnecessary tension is removed, so my technique is better and I miss fewer notes in super difficult passages.
When I come out of an AT lesson, I feel fantastic and floaty and at least 3 inches taller.
Aha! That is really helpful. I looked at the website and didn't fully get it. Thanks for explaining. I will take a look at it. I hold my tension consistently in some areas of my body (shoulders) but this has expanded recently. It sounds really helpful.
In other news, I have been working with the tennis ball to the shoulder...it has been really amazing. I can't wait for my real massage though!