X-Sync is a photography term that describes the fastest shutter speed at which the whole imaging sensor in a camera can be exposed to light entering through the lens at once. Canon DSLR’s generally have an X-Sync of 1/200 or 1/250. At speeds faster than X-Sync, the imaging sensor is exposed sequentially from top to bottom.
This has implications in Flash photography; for example, as only part of the imaging sensor is exposed at the time that the flash fires, only that part will be properly exposed in the photo, with dark bands in the other areas of the frame. As a safeguard against this unwanted behaviour, the camera will only operate up to the X sync speed, if a non-Canon external flash is attached. To shoot with a Canon Speedlite flash
at faster shutter speeds, you need to enable ‘High speed Sync’, which modifies the behaviour of the shutter mechanism and flash.