-If specified, only specific plugins are flushed. Otherwise all plugins
-providing a flush callback are flushed.
+If the B<identifier> option is given only the specified values will be flushed.
+This is meant to be used by graphing or displaying frontends which want to have
+the latest values for a specific graph. Again, you can specify the
+B<identifier> option multiple times to flush several values. If this option is
+not specified at all, all values will be flushed.