I thought my D610 says 1/250 sec? What is HSS? The camera setting allow Auto FTP or something like that which say 1/320. According to the YouTube video, it can adjust to higher speed. I have yet to try.
To answer your query, you gave to understand how a Focal Plane Shutter works
. There are two parts to the shutter - the front curtain and the rear curtain.
. When the shutter fires the front curtain starts travelling. Depending on the shutter speed the rear curtain follows after some time. So in effect the shutter exposes the sensor in a strip. The width of the strip determines the amount of light reaching the sensor - in effect the exposure time. If the exposure time selected is more than the time required for the front curtain to reach the end, then the rear curtain follows after the exposure time is reached.
. The electronic flash fires a very short burst of light, normally between 1/5,000 and 1/15,000 of a second. So the shutter has to be fully opened for the flash to illuminate the object and the sensor to register it.
. For normal flash operation the front curtain has to travel from top to bottom (or one side to the other, depending on mechanism). The rear curtain stays put. If the speed selected is lower still the rear curtain will fire after the selected time.
. There is a limit to the curtain speed, and that is the maximum shutter speed at which a flash will illuminate the whole scene. That in effect is the sync speed quoted.
HSS or High Speed Sync is a combined capability of both the camera body and the flash to send a signal to the flash and the flash to fire at regular intervals so as to illuminate the scene in strips (synchronizing with the shutter opening that is travelling). As the normal flash take some time to power up to full strength, during HSS the flash power is reduced so that it can supply uniform light for all those bursts (in effect the flash has to calculate the number of bursts that are needed for the duration of the shutter movement, then determine at what power it needs to operate to deliver uniform light across the bursts). The faster the flash can charge the more power can be delivered during HSS.
HSS also depends on the implementation in the body. D70 with CCD sensor could HSS at 1/4000 or faster.