Optional Excerpts

I can’t for the life of me figure out how “optional excerpts” work. There is a particular effect I would like to obtain, and no matter what I read on I can’t seem to find out just how it’s done.

What I want to do is make it so a post on the front page will only display what I’ve put in the excerpt field, then display the proper post when a user clicks on the title or the “read the rest” link. I don’t mean this in the way the <!–more–> tag works, either.

I was wondering if it was possible to do this, but still have the regular content show if I use the <!–more–> tag instead, and the excerpt is empty. I’ve seen a site doing something similar to what I want to do, you can see for yourself [here].

Thank you very much to anyone who can help.