Back Lighting within the confines of a room will fill with light faster than outdoors. In contrast this same room will take more time to fill than a soft box. This is a rather obvious statement I know, however I must often remind myself of this fact within the context that light moves at the speed of 186,000 miles per second. I still have a hard time truly comprehending that number.
Perhaps this will help to comprehend the speed of light from our flashes to think of light in miles traveled in 1 second contrasted to the circumference of the earth. 
Light traveling at 186,000 miles per second around the earths 25,000 miles (rounded to the nearest thousand for simplicity) will lap the earth 7 times and then some in 1 second! Is that fast enough for you?
The theory of Back Lighting has more to do with the saturation of light into the camera lens than light’s speed, so I will not distract us with more facts. I do want to impress you with the speed of light in that light is reflected, refracted and split again throughout the staging room while your shutter is open. So position your flashes and other light tools wisely so as to capture the most light as possible with you camera.
In your style of Back Lighting, do you want to see the light source? Place so as to draw the viewers focus to you subject. The picture above unintentionally captured the light and movement of a passing vehicle, so it is not as I would have set-up the shot, but is a good demonstration of being spontaneous in life’s unique moments. 
The picture below illustrates my point of using the light source to draw a viewers focus toward the subject. The light from the moon is vignetted in a spreading ray toward my subject while at the same time Back Lighting him in dramatic effect.
The first photo in this post also uses some techniques of Back Lighting without silhouetting my subject. Silhouetting is most often used to emphasize the edges or shape of the subject (whether it is person or object). This is done in both pictures: Picture 1, by illuminating the side of the subject and letting the shadow on the other side disappear into the surrounding background. Picture 2, is contrasting the head and shoulders of the subject against the semi-illuminated sky in the reflected light of the moon.
Enjoy these techniques. Get out and play with long shutter releases and beautiful skies!
Foe-toss | LIGHT 
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