This is a GeoGebra version of the classic Peaucellier Linkage which draws a line without a straightedge. For a physical version, please refer to http://www.youtube.com/watch?v=hSdW-i3nO1M . What does it take to build a GeoGebra simulator?
(1) What is the inverse of a line with respect to a circle? (2) Take a look at the produt PA * PD. Does it change at all when A moves around?