-Updates the index and working tree to reflect the specified branch,
-<branch>. Updates HEAD to be <branch> or, if specified, <new_branch>.
+
+When <paths> are not given, this command switches branches, by
+updating the index and working tree to reflect the specified
+branch, <branch>, and updating HEAD to be <branch> or, if
+specified, <new_branch>.
+
+When <paths> are given, this command does *not* switch
+branches. It updates the named paths in the working tree from
+the index file (i.e. it runs `git-checkout-index -f -u`). In
+this case, `-f` and `-b` options are meaningless and giving
+either of them results in an error. <branch> argument can be
+used to specify a specific tree-ish to update the index for the
+given paths before updating the working tree.
+