The past weekend, I wanted to copy a two-light setup: Key light and Back/Rim Light. I only have one flash on-hand that made it a little tricky. I'm really thankful for the Internet -- it's easy to get information on this topic, actually on any topic. I was able to read lots and learn lots of techniques on how to do it and thus the first photo:
Okay, I used the sun as my back/rim light and the flash to light up the subject's face. The back/rim light acts as a separator between the subject and the background. Here's another photo from my experiment on the technique.
If you will see the back lighting on the hair that made a separation on the dark background add dimension to the photo. If the back lighting on the hair was not present, the subjects head would tend to blend in with the dark background.
I'm looking forward for another session on this experiment.