I had the same problem!
I know the picture in the book has her with her body almost upside down, not far from a jackknife pike (
http://www.youtube.com/watch?v=PIx4JGxwL8c).
I was having a really hard time with the exercise too, until I realized you can do them like this:
http://www.youtube.com/watch?v=jj0eox45Dv4
Sooo much easier to manage, but still hard work for the core. I typically fall off the ball from exhaustion on my 10 rep or so

Hopefully one can build up to the more advanced versions.