X is for... X- sync speed

The x-sync speed commonly called sync speed, is the fastest speed at which the built-in flash will synchronize with the camera. and it varies by camera model. When using High-Speed Sync (HSS) flash with Canon EX Speedlite flashguns, where the shutter speed is faster than the x-sync of the camera you'll find that the flash power is greatly reduced. If you need to shoot at faster shutter speeds, for example so that you can use a wide aperture to achieve a shallow depth of field, or to freeze the motion of a fast-moving subject. then as the shutter speed increases you'll find that the flash power continues to fall off the faster you go. To overcome this, if you need a lot of light the solution is to use more flashguns. For example, if the flash power drops to half you'll need to use two flashguns to have the equivalent of one full-power flash; if flash power drops to a quarter then you'll need to use four flashguns, and so on.

X is for... X- sync speed