One of the rules in photography is not to take a photo with the light behind the subject but there are instances when breaking that rule produces great results. Called silhouettes, it is when the main light, natural or artificial, is behind the subject.
Here is a photo of a beautiful bride as she was about to enter the chapel. The main light which was the sun was behind her and I used an off-camera flash with powered-down setting for fill light. It was just enough to enable us to see part of her face and the design of her wedding gown. Using no fill flash would had made the subject total in the shadow leaving us with just the contours of her feature.
Here is another photo of the same bride but with the fill light adjusted for higher power than the previous photo, just enough to show us more of the features of the bride and her dress but still leave some mystery.
In these two photos I used an off-camera flash for fill light but there are other things that can be used to light the subject. A reflector is one though using a white illustration board or styrofoam would be cheaper if you are not yet prepared to purchase a 5-in-1 reflector.
I hope that with these tips the next time you are in a situation that your subject is behind the main light, you would not hesitate to click your shutter button.
Photos taken with a Canon EOS 70D with a Canon EF-S 18-135 mm f/3.5-5.6 kit lens.