git-svn: What is --follow-parent / --no-follow-parent for?

Hi,

on reading the docs of "git-svn", I stumbled across this paragraph:

--follow-parent
This is especially helpful when we’re tracking a directory that has been
moved around within the repository, or if we started tracking a branch
and never tracked the trunk it was descended from.  This feature is
enabled by default, use --no-follow-parent to disable it.

However, this does not make sense to me: This sounds like there is no
good reason *not* to enable this option.  So why is it there? And in
what situation might I want to use "--no-follow-parent"?

As a matter of fact, I'm not even sure what "--no-follow-parent" does
(and the docs don't really say). 

I tried it out with a small test repo with a single branch (produced by
copying the trunk, then later deleted). With --follow-parent git-svn
correctly detected the branch point, and modeled the branch deletion as
a merge. With --no-follow-parent it just acted as if branch and trunk
were completely unrelated.

Commit graph of git-svn result:

--follow-parent:               --no-follow-parent:


       |                               |
      /|                             | |
     / |                             | |
     | |                             | |
     | |                             | |
     | |                             | |
     \ |                             | |
      \|                             | |
       |                               | 


(please excuse cheap ASCII art)

Is that the only effect of --no-follow-parent? And again, why would I
want that?

I'd be grateful for any clarifications. If I manage to understand the
explanation, I'll volunteer to summarize it into doc patch (if there are
no objections).