"--track" in linkgit:git-branch[1] for details.
+
If no '-b' option is given, the name of the new branch will be
-derived from the remote branch. If "remotes/" or "refs/remotes/"
+derived from the remote-tracking branch. If "remotes/" or "refs/remotes/"
is prefixed it is stripped away, and then the part up to the
next slash (which would be the nickname of the remote) is removed.
This would tell us to use "hack" as the local branch when branching