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

  If you perform a Go To Topic operation on the
  selection, the current help topic in the Help window is
  replaced by the new topic.

  In the Help system, performing a Go To Topic operation
  is the most common operation. So in Help, selecting
  something from the list of additional topics by double
  clicking MB1 performs a Go To Topic operation.

  See the Visit Topic menu item for the alternative to
  Go To Topic.