This is the simplest and the least “charming” of all. It simply toggles your current screen and the Start Screen.
How to Invoke It
It basically acts as the traditional Start Button. The Start Button invokes the Start Menu. Similarly, the Start Charm invokes the Metro Start Screen (Windows 8 incarnate of Start Menu).
When you invoke it again, it will return you back to the screen that you were most recently using – provided that you had anything open. Otherwise, it will just stay at the Start Screen.
This charm may be the most redundant and unnecessary one. The effort to invoke it is more than the result it produces. I personally think that it has another, more important goal: to show the new Windows logo in all its glory.