Could someone explain what is happening in falsetto that's different from other types of singing? Is it different vocal chords being called into service? Or something like over-blowing in a harmonica?

I can't really see how "natural" and "artificial" comes into it. With all types of singing, if you can do it, it feels natural, if you can't do it, it's impossible, and if you learn how to do it when it used to be impossible, it's artificial.