A lot of photographers like to use a shallow depth of field for portraiture; however, if you are lighting your subject with flash, you will be limited by the flash sync speed of your camera. There are ways around this, however: high-speed sync and ND filters. Both come with their own pros and cons, and this excellent video discusses which is better for your work.
Coming to you from Daniel Norton Photographer, this helpful video discusses using ND filters versus high-speed sync. The need for these arises from a decades-old issue: flash sync speed, which dictates the fastest shutter speed available to your camera during which the entire sensor or film is exposed at the same time, which thus places an upper limit on the shutter speeds you can use with a flash, normally somewhere around 1/250 s. When you use wide apertures for shallow depth of field, this can quickly cause a problem, since you will often need faster shutter speeds. This is where ND filters and high-speed sync come into play. Personally, I prefer the convenience of high-speed sync, but it comes with its own set of drawbacks, and other people prefer ND filters. Check out the video above for the full rundown from Norton.