A cool trick that you can do with an external flash is to take a long exposure shot, preferably in the dark (basically your aim is to light only the subject with the flash, there should not be anything else in the background to catch the light) and fire the flash multiple times on your subject while the exposure is taking place. You will end up with multiple instances of your subject.
Most modern flash units have a mode called Multi or Repeating Flash mode in which you have options to adjust variables such as the output level, how many Times it flashes, and the Frequency rate in Hertz.
|Stroboscopic Photography Effect|
The same picture as above, converted to Black and White
For a detailed tutorial on how to do stroboscopic flash shots refer this post which we published earlier in our website. Stroboscopic Flash Photography .