Forcing use of a particular index

http://www.postgresql.org/docs/8.0/static/indexes-examine.html

Says:

"If you do not succeed in adjusting the costs to be more
appropriate, then you may have to resort to forcing index
usage explicitly."

Is there a way to force a query to use a particular index?
If not, what does this sentence mean?

Thanks.


Karl <kop@meme.com>
Free Software:  "You don't pay back, you pay forward."
                  -- Robert A. Heinlein