High-speed sync is a special flash mode that can overcome the limited flash synchronisation speeds of focal plane shutters. Flash duration is typically very short, but focal plane shutters can only achieve higher shutter speeds by exposing the sensor through a moving strip rather than all at once. High-speed sync modes typically ‘pulse’ the flash several times during the time the shutter is open so that all of the sensor is exposed equally.
This usually comes with a loss in effective flash power, though flash maker Elinchrom has developed a ‘High Sync’ mode that uses carefully adjusted flash timing and duration to minimise the power loss.