I see them used in one of the demos.
animation can either be set to 'slide' or 'fade'.
start is a callback, which is triggered when the slideshow starts.
These are both documented in the jquery.flexslider.js file at the bottom section of the file, as well as in the README.md file and at http://woothemes.com/flexslider/. :)
Thanks for the quick reply. I was looking at https://github.com/woothemes/FlexSlider/blob/master/README.mdown, which I thought was the documentation.