HSS is generally an open-loop process. The camera and flash use predefined, manufacturer-specific timing so the flash emits a rapid series of pulses (or effectively near-continuous light) while the shutter slit travels across the sensor. It is not timed by reading light off the sensor between pulses.

Because of that, HSS works in complete darkness just fine. It does not need ambient light or sensor feedback to know when to fire the next pulse.

Also, the system does not depend on the light dropping to zero between individual pulses in a way that would create exposed/missed horizontal bands. In practice, HSS behaves more like a very fast pulsed light source during the shutter transit, which is why it can cover the moving slit evenly.

A tradeoff is reduced maximum flash power compared with normal flash sync, since the flash has to spread its energy across the whole shutter travel instead of one short burst.