First, what is happening inside your DSLR?
Before going into the second curtain sync mechanism, it is necessary to understand the camera and what happens when you press that shutter release button. To some of you, you may know this already and it might be a boring topic for you, but I suggest you play that video I found on youtube showing the mechanism at different shutter speeds. It is really amazing how engineers managed to design such a complex mechanism.
Next I have also found this diagram to the right illustrating the moves of both the first curtain and the second curtain while capturing the exposure on the censor / film.
The diagram below shows where the shutter sits in your DLSR. The red line being the sensor and the blue and green lines are the leafs of the first and second curtains respectively. When you press the shutter release, the mirror flips out of the way and the curtains start to roll in a synchronised manner with your camera settings.
But if you were like me, I used to believe that the DSLR has only 1 curtain roll that drops down then up quickly. Well this is not the case because as shutter times get faster and faster, the mechanics of a the mirror and one single curtain fail to keep up with both dropping out of the way and springing back up. This is why two curtains are employed.
Try this out; set your shutter to something more than one second, focus on anything and hit that shutter release. You will hear the initial slap of the mirror moving up, the first curtain click, then after the one second or more, you will hear the second curtain followed by the mirror dropping down and the whole mechanism will reset.
When things get faster, things look like the first and the second curtain moving in parallel.
Now back to the "Second Curtain Sync"
With the help of Jad (my son) who agreed to pose for me to write this post - thank you Jado - I was able to show you both photos taken with the same exposure settings, but the first one (Photo 1 below) with the flash synched to the second curtain and the second photo (Photo 2) with the flash synched to the first curtain. Notice the difference in the car trailing lights.
What was happening is directly related to the above video and diagram. Basically the flash was on my camera hot shoe with 1/16 of its power. With a slow shutter speed, to create the effect of trailing lights in the background, setting your flash to the proper sync makes all the difference. Notice in Photo 2 how the moving object comes after the blur caused by the car stop light.
If you want the car stop lights to be behind the car, go to your flash control settings on your DSLR and change the default to "2nd curtain" sync (on Canon). Technically the flash will fire just before the second curtain i.e. just before the exposure is ended (Photo 1). If my flash was set on the first curtain sync, it would fires immediately with the first curtain and then allows the object to continue moving before closing the second shutter, ending the exposure (Photo 2).
There you go, explained with examples, the Second Curtain Sync requires little practice, and sometimes luck. Set your shutter speed to less than 1/10 second, wait for the passing cars in the background, and take the shot.