Last week I had a studio shoot where for some shots, I wanted to drag the shutter (use a slow shutter speed) and pop the flash at the end of exposure (called rear or second curtain sync) instead of at the beginning (called first or front curtain sync), which is the default. This is easy to do if you're using Canon speedlites; all you have to do is set the EX strobe into 2nd curtain mode, which is designated by a little triple arrow icon. However it gets a little more complicated if you want to use studio strobes.
Problem
The 1Ds series Canon cameras have a custom function setting (C. Fn II 5 on my EOS 1Ds Mk III) that lets you toggle between 1st-curtain synchronization and 2nd-curtain synchronization. One would logically think that this is all you have to do, and from then on, any strobe or wireless trigger connected to the hotshot or to the PC sync port would fire on the 2nd curtain. Alas, it's not so. I thought this was a bug, or user error, but after talking to Canon tech support I learned that that's not the case. That command only applies to Canon Speedlites, and since the 2nd-curtain sync function is built into most (all?) Speedlites, the option on the camera is essentially redundant. Which probably explains why it's not even in the 5D/7D series bodies.
Solution!
So what's a photographer to do? My Profoto studio strobes aren't sophisticated enough to include the rear curtain sync option, so I thought I was out of luck—but the Canon tech rep had a great suggestion. Most studio strobes have optical slaves built in, and if they don't, that's a really cheap accessory you can add. So instead of relying on a wired or radio wireless connection to trigger the studio strobes, put an EX light on the Canon in rear-curtain sync, and let that be the trigger for the studio strobes. Even at the lowest power setting, and pointing up to the ceiling so it wouldn't light my scene at all, the EX triggered my Profoto studio strobes every time. Voilà, rear curtain sync with studio lights.
This may not work in every situation, such as where the studio strobes are so far from the camera that they can't be triggered, or too far out of line of sight, but I'd wager in most studio situations, this would work quite fine.
Also keep in mind that if the on-camera strobe is adding light to your scene (maybe the camera is extremely close to the subject), you could put the EX on a TTL sync cord, and get it away from your subject.
Example
In the photos below, you can see the difference between front and rear curtain sync for what I was doing. Or just google it for some very clear examples.
Here's an example of first or front curtain sync. Her face is frozen at the beginning of the exposure, then she moves through herself. Weird.
With second or rear curtain sync, the movement shows with the model frozen at the end, which is usually a more pleasing and "expected" view.
Rear curtain is almost always the way to go, and if you want to do it in the studio with "big" lights… now you know how!