Well you've proved that hi speed flash sync works and it is indeed a modern miracle. You didn't actually need it for this shot, but there's nothing wrong with the enquiring mind and a little experimention.
Hi speed sync works by putting out a continuous pulse of flashes rather like a string of beads. It isn't true sync at all, it puts out this pulse of flashes thoughout the shutter duration, because when you select a shutter speed faster than the max flash sync speed of your camera, those speeds are achieved by the two shutter blinds moving together with a slit between them exposing the whole frame incrementally - the faster the shutter speed, the smaller the slit. Hi speed flash sync provides light to the scene for the whole time that slit is moving across the sensor.
It's best used fairly close up because as the range increases the larger the aperture you'll need to use. Increased range or smaller apertures soon put a demand on the flash which it can't meet.
It can produce some spectacular effects though, macro being a good example, where you can light an insect on a leaf (for example) quite perfectly, with a light coloured background immediately behind it being reproduced deep black.
