It's not that straightforward. Here's what I've noticed - if you type in a search word that has 4 or more characters, you get posts that actually have those characters in them. But, if you type in a 3-letter search request, it looks like you just get all the recent posts, even posts not containing those characters. Try it and click on some of the posts it returns, not all of them have the requested 3-letter string.
For example...try searching for "emulsion" in the pine forum. You'll get a bunch of posts that actually have the word in them. But then try searching for shorter versions of the word. If I try "emul" I get no returns! Why not? If I try "emu" I get a lot of hits but most of them don't have "emu" in them. Instead, it seems to simply return every recent post in that forum.