After you have selected a topic from the list of
  additional topics, you can perform a Go To or Visit
  operation on that selection. If no topic is selected,
  these menu items are dimmed. Double clicking on the
  selected topic achieves the same result as selecting a
  topic and then choosing the Go To menu item.

  If you perform a Go To operation on the selection,
  the new topic replaces the current topic in the Help
  window.

  Contrast with Visit Topic.