This is sometimes the toughest question for me to answer. The overall goal never changes, but the finer points are very elastic, so it's a challenge to stay focused and be articulate about what is happening and why.
Here is my latest attempt to wrap up a tight explanation. I hope you enjoy it.
This is a flash presentation, you have to click the arrow and give it a second to load. Then move your cursor over the word "more" and select "fullscreen" It is best to click through at your own pace. (In other words don't use the "Autoplay" option. )