On my new quest for a 100 series, I found this paper explaining how A-TRAC works. Kinda cool if you are an engineering geek like me.
http://classes.engr.arizona.edu/engr498a/2006/fsaediff/resources/techart3.pdf
http://classes.engr.arizona.edu/engr498a/2006/fsaediff/resources/techart3.pdf