For many portrait and wedding photographers, high-speed sync is a savior, but for those new to lighting, it can be a bit of a strange bird. Chicago-based Photographer Manuel Ortiz has created a great video that demonstrates why HSS is a desirable thing to have.
In short, high-speed sync allows your camera to shoot at faster shutter speeds than its normal flash sync speed by essentially approximating a constant light source through the use of many very fast pulses. This allows you to shoot at those wide-open apertures that many photographers love, but still control your ambient light. You might ask why one doesn't use an ND filter, which many photographers have done for years. This is still a viable option, but it can make it harder for you to see your subject if you're using a DSLR, and can also make it difficult for the camera to focus, depending on how much light you're blocking. HSS eliminates this problem by allowing you to shoot normally at any shutter speed you please, meaning you can get those beautiful shallow depth of field shots with a proper balance between flash and ambient light.
[via DIY Photography]