Evil,
I was going over the pictures and text. It looks like the ack angle is the difference between the outside and inside wheels angle. So if the left wheel turns in 20deg and the right wheel turns in 30 deg, then the ack angle is 10deg. The larger the angle makes for smoother corner according to the text. I am by no means an expert. Like you I am trying to get a handle on this stuff. The website I am referencing is http://users.telenet.be/elvo/