One of the most useful features for flash photographers is high-speed sync, which allows us to use flash at essentially any shutter speed we would like. This excellent video shows how a focal-plane shutter works and why that creates the need for solutions like high-speed sync.
Coming to you from Andrew Boey with Beyond Photography, this excellent video will show you why high-speed sync (HSS) is needed for flash photography. High-speed sync (and similar technologies) are necessary because of the fundamental limitations of a focal-plane shutter, which works by drawing two separate mechanical curtains across the sensor — the first to begin the exposure and the second to finish it. After a certain shutter speed, there is not a time during which the entire sensor is exposed simultaneously, which means the essentially instantaneous pop of the flash cannot illuminate the entire sensor area, thus the need for high-speed sync. HSS has a few of its own drawbacks, such as loss of power or slightly uneven exposures, but overall, it is a fantastic solution for a lot of situations. Check out the video above for the full rundown.
If you would like to really dive into learning lighting, be sure to check out "Illuminating The Face: Lighting for Headshots and Portraits With Peter Hurley!"