Normally, when a n electronic flash fires, it fires at the start of the exposure. Think of a 3 second exposure... click the shutter button, the shutter opens the flash fires and... and... and 3 seconds later the shutter closes.
Here's golf ball rolling down a slope at 1/4th of a second with flash:
ISO 800 f7.1 1/4
In Canon's language, this is known as 1st Curtain Sync. There is also 2nd Curtain Sync, where the flash fires at the end of the exposure:
ISO 800 f7.1 1/4
In the first image, it looks like the ball is rolling up hill ! The second image is much more pleasing.
Just for fun, the Yongnuo 568EX flash also has a "Multi" setting (a stroboscopic effect):
ISO 800 f7.1 1/4
The flash settings were 4 shots at 12Hz:
Have fun