Some ﬂash units have a high speed sync mode, that allow a camera to take a photograph with the ﬂash while using a shutter speed that exceeds the X-sync speed. The photo won't appear with improper light levels. How might this high speed sync mode work?
Each camera with a mechanical shutter has a speed that is the fastest it is capable to sync with a flash. It is usually around 1/200 to 1/250 sec, but can be much faster or slower depending on the camera. At speeds faster than this the second curtain of the shutter begins to close before the first curtain is completely open. The sensor (or film) is not being exposed all at the same time, but instead is being exposed from top to bottom (or side to side for most older film cameras) by the opening between the two curtains. The faster the shutter speed, the narrower the gap between the first and second curtain.
Since an electric flash strobes at a very short duration, only the fraction of the sensor that is behind the slit between the two curtains will be exposed to the light from the flash, and the top and/or bottom of the frame will have dark bars across them. The solution when flash is needed at a high shutter speed is for the flash to fire a series of bursts while the curtains move across the sensor. This means the flash must fire several times in very quick succession. To have enough power for that many pulses of light, each one must be weaker than a single, high powered burst. Each pulse is dimmer, but because the flash is pulsing many times, the total power used is relatively high in most cases.
The problem with a shutter "faster" than the sync speed is that the first and second shutter are both partially closed at the same time. Thus, there is no moment during which the flash can fire and expose the whole sensor. High speed sync works by firing the flash multiple times as the shutters expose small portions of the sensor continuously. It gives up a lot of flash power since the flash has to fire repeatedly in rapid succession, but it will allow an even flash exposure faster than the shutter can normally operate for being fully open (the sync speed).