Inspired by this portrait, here's my attempt at backlit portraits.
The only difference was that, in my case, there was *no* ambient light. And since I have only one speedlight (SB600) I had to reflect some light from the sb600 on the model's face. The technique is fairly simple. There is a powerful light source behind the subject that gives the backlight, there is another light source (or ambient light) that lits the subject. In my case I placed my SB600 behind the model. There was a white wall on camera right that reflected the light from the SB600 onto the model's face. The SB600 this provided both the backlight as well as the light that lit the model's face. Since it bounced off the wall, the light was soft. We shot in the terrace at night, so there was no other undesired light in the frame.
Adding to that, here is some information about the post processing. I added some diffuse glow and a bit of gaussian blur to the image to add to the softness. The colors and light are natural however.
Edit: I forgot to add that I used a CactusV2S wireless trigger to trigger the SB600.