-git-rebase replaces <branch> with a new branch of the same name. When
-the --onto option is provided the new branch starts out with a HEAD equal
-to <newbase>, otherwise it is equal to <upstream>. It then attempts to
-create a new commit for each commit from the original <branch> that does
-not exist in the <upstream> branch.
+If <branch> is specified, git-rebase will perform an automatic
+`git checkout <branch>` before doing anything else. Otherwise
+it remains on the current branch.
+
+All changes made by commits in the current branch but that are not
+in <upstream> are saved to a temporary area. This is the same set
+of commits that would be shown by `git log <upstream>..HEAD`.
+
+The current branch is reset to <upstream>, or <newbase> if the
+--onto option was supplied. This has the exact same effect as
+`git reset --hard <upstream>` (or <newbase>).
+
+The commits that were previously saved into the temporary area are
+then reapplied to the current branch, one by one, in order.