Bod, the flash only fires once whether you use first or second curtain sync.
The function is only really useful for shots where objects are passing across the field of view at right angles (more or less) to the camera.
Take this shot in which first curtain sync (normal) sync has been used.
Imagine the camera on a tripod and the girl with the sparkler running across at right angles from right to left. As she comes into shot and you fire the shutter, the first curtain opens, exposing the sensor, and the flash fires. The flash lights up the girl and records her on the sensor straight away at the right edge of the frame. The duration of the flash is extremely brief and dies long before the shutter closes. During this period, she continues to run across the frame but isn't bright enough to record on the sensor, but the sparkler is, and lays that light trail across the sensor. The final result is really strange because it appears she's running backwards. You get the same effect with car lights and flash in the dark.
Take this shot in which 2nd curtain sync has been used..
Now the first curtain opens fully, exposing the sensor, but the flash doesn't fire yet. The girl is crossing the frame but doesn't record because it's dark and she's moving too quickly. The bright sparkler though records it's trail across the sensor. Now just before the second curtain closes to end the exposure, the flash fires, freezing the girl's motion but this time on the left of the frame with the light trail behind her.
That's all there is to it.
