What is Hi Speed Sync?

The Basics:  

High Speed Sync enables you to shoot with an external flash beyond the camera’s X-Sync Speed.

The way it works is that the second shutter curtain starts to close before the first shutter curtain has fully opened. Instead of having the entire sensor exposed at once therefore, this creates a narrow band that exposes the sensor sequentially from top to bottom.  

With High Speed Sync enabled on the Canon Speedlite, the flash emits a continuous pulse of light, instead of one quick flash. This enables the entire imaging sensor to be exposed correctly to the Speedlight’s light. 

The benefit of High Speed Sync with Canon’s Speedlites is that you can use fill flash, and a large aperture on a bright sunny day, thus preserving your creative intentions.