For some time now I have been experimenting with remotely triggered flash guns to achieve different effects and I was at a friend's daughter's Christening the other day and I decided try and capture some people dancing against back light. In this case I used a couple of Canon flashguns, one on the camera as the master and another placed on the disco stage in slave mode. Admittedly this is an expensive solution but for about £20 you can buy a Chinese wireless transmitter and receiver on Amazon (Yongnuo RF-603 and similar) and achieve the same effect.
For this picture I noticed this girl was jumping to the music while holding a balloon. The flash has illuminated the balloon and the outline of her hair, and by underexposing the rest of the image she becomes just a silhouette. Lastly, I placed an orange gel filter in front of the flash gun so the colour of the light was in keeping with the rest of the scene. (flash guns tend to produce rather blue light when compared to indoor lighting).
The classic application of this technique is for a bride and groom's first dance.