Where can I find a good description of that exercise?

I think you'll find much more help asking in http://thetaobums.com/

I found that "Basic Theories and Methods of Zhineng Qigong" by Yuantong Liu has a detailed description.

There is a description of the first 2 levels under methods at http://www.zhinengqigong.org/

The Voyage to the Shore Trilogy by Yuan Tze at http://www.yuantzerenxue.org/materials-books/ (or amazons)
is about Zhineng Qigong but would have to be one of the best qigong books around

cheers
Jeff