I've been asked a few times lately about high-speed sync - what it is, and how it works so I'm going to have a go at explaining what it is and how it works.
The simple answer to the latter is that I've absolutely no idea (it's far too techy for my meagre brain to understand), but I do know how to get it to work for me.
The theory behind high-speed sync is that most strobes will sync with a camera at a given shutter speed, which is generally around 1/250 second.
If you were to put your camera and flash into full automatic, you will find that in brightly lit situations your camera will set itself to this highest shutter speed and go no further.
What this means, is that often, especially in bright situations you'll find that you cannot get your shutter speed high enough to expose properly, and avoid blowing out the highlights leaving you with a horribly over-exposed image.
Why would you want to use a flash in this situation you might ask. Well, if you want to create a little drama in your image say, then you'd ideally like to expose the background or sky properly, or even under-expose a little and still have your foreground / subject adequately lit. Without an additional lit source, be it a strobe or a reflection of some kind there is no way that you'd be able to do this. So say that you are shooting a portrait outside in the middle of a typical sunny day, and you want a nice shallow depth of field, then you could be shooting at around 1/4000 sec at F2.8 for example to get the background properly exposed. If you then place a person with their back to the sun, then their face wouldn't be getting anything like the same levels of light and would appear under-exposed. You'd need somehow to throw a little light back onto the subject. You could use a reflector, but this may not create the look you're going for, or may not be possible. So your other option is to use a flash, but if the fastest your strobe can sync with your camera is 1/250 sec then you're screwed.
This is where high speed sync comes in. With Nikon DSLRs it's as simple as going into your camera's flash settings and enabling Auto FP mode (Menu > Custom Settings > Bracketing / Flash). This will allow you to fire your strobes at any shutter speed, and instantly solve the problem of the scenario above.
So now let me introduce you to Keith and Helen. My kids two best friends, who has you can see have been well loved. After a conversation with an art-director last week, this image popped into my head so I figured I'd give it a whirl. A little lacking in space, I decided to shoot this on my office sofa. I wanted the only light on the teddies to come from my flash so that I could sculpt it as I saw fit. An initial test found that to kill the ambient light coming through the windows and still keep a little depth of field, I needed a shutter speed of 1000/sec at F5.6. Using Auto FP sync mode, meant that I could fire an SB900 wirelessly using TTL by simply using the pop-up the flash on my camera. And that's it - I have total control over the light in the photograph.
With an SB900 in a 30" Lastolite Ezybox hotshoe just a few inches above my "subjects" and a silver reflector on the other side just to throw a little of the light back into the shadows on the other side, I had a perfectly lit "portrait"!
For more information on high-speed syncing and off-camera flash, here are my two main go to resources:-