While I was doing some research in the fulltext indexes, I read in forum search help:
"For basic searching of one or more words anywhere in the texts, just type them separated by spaces. All words longer than two characters are used"
And i assumed it was true and mimic such behaviour in the fulltext indexes (slow behaviour!).
Some minutes ago I was performing some searches against moodle.org without fulltext indexes and I discovered that relational search is looking for ALL (AND between expressions) tems, no ANY (OR) of them.
So I think the help file should be fixed (and I'm changing fulltext behaviour to this mode that will be really faster!