Assuming you have a local git branch "1.0" which is where you "git svn rebase" and "git svn dcommit" for an SVN branch, a common use case would be:

1. git checkout -b new-1.0-fix 1.0
2. hack hack hack, commit commit commit
3. post-review --gitsvnparent=1.0

The reasons why I need a feature like this are best described in the bug.