Right now, this feature is undocumented and I do not have time before feature freeze to provide documentation or support for it. So, I am not publicizing it widely because I do not want to attract testers except for those testers who can figure out how to use it by reading the code. Later, when it is more complete and better documented, it would be more useful to have testers.
EDIT: There is now documentation at Schematic Editor | master | English | Documentation | KiCad