Ferret talk - May 2009 - How does it score?

Hi,
how do I find out what ferret actually does? My program works fine:

qp = QueryParser.new(:default_field => :title)
index.search_each(qp.parse("Some words"))

Now it seems that this way not all terms are required, but how does  
that affect the score?

I didn''t find the algorithm in the documentation, and I''m no
good at
reading large C projects.