A dual helix is the shape of a compact florescent bulb.
This is a gif, please open in preview so you can go frame by frame.
Hey ZOO... nothing happens when I attempt to "Join Segments"...
No connection is created, like what is shown in your GIF.
Edit: Your GIF does not say/show selecting the upper point THEN selecting "Join Segments"... but I got it now!
Moving on!