It works best for thread titles, not post titles.
In general, Google likes:
- Titles of pages with keywords
- Links with keywords in them pointing to a page
- Keywords in the page address
- Headers, paragraphs, main text with keywords in
- Internal links with keywords in
- People staying a long time on a site before returning to search results
- Recently updated pages
- A domain name that's been registered a long time
- Images with keywords in the alt tag or description
- Links from well-known websites that are known to be reliable (eg Wikipedia, BBC and so on)
- ...and a bunch of other stuff
So, because titling a thread well creates links to that page from the main forum page, plus an address with keywords in, plus a page title with keywords in, Google likes it a LOT.
Hope that's helpful and not too technical.
Rachel xx