New feature on Google search results

I’ve noticed recently a new feature sporadically appearing on Google search results:

As you can see, the BBC News website is returned here as an example, but with what must be “popular secondary links” included underneath the page summary.

This only seems to appear on certain requests, presumably when the request is served by a specific server(s)? Refreshing the same search result may or may not return a set of results that may contain these “secondary links”.

This seems to raise a few questions (with thoughts in italics):

1. Why does it only appear on certain searches? (is it a beta feature rolled out to a small farm of servers?)
2. When it does appear, why are there only “secondary links” for certain results – ie why BBC and not CNN in this case?
3. How does Google decide what secondary links to pull in? (Google have started measuring click throughs, recently – is it based on individual page popularity or Google Rank?)
4. Why have they added this feature? (Is it to boost the pre-fetching Google can do on Firefox browsers?)

Leave a comment if you can shed more information!