Why would you want to use high speed sync with a Canon Speedlite 580EX II Flash?
High shutter speed synchronization with your Canon flash gun gives you the ability to use and control in amount of fill light on your subject in brightly lit situations.
The ability to use a high shutter speed with flash photography is not understood by many photographers and is used by even fewer. That's a shame because it can be very useful in several situations.
Because of the way a focal plane camera shutter works, there is a limit to the maximum speed of the shutter that still synchronizes with the firing of your flash. Maximum sync speeds are usually around 1/60 second for entry-level cameras, up to 1/250 second for high-end cameras.
In "standard" flash mode, the flash fires for one brief blast of light usually for 1/800 of a second up to 1/50,000 of a second for very low power settings. Flashes with high-speed sync capabilities actually fire the flash hundreds or thousands of times per second.
This is so fast that we can't even see the stroboscopic flashing. It actually acts like a continuous source of light the entire duration that the camera shutter is open. All of these short pulses of light allow the camera shutter and flash to expose your image during the entire duration of the opened shutter.
When you have strong back lighting your flash may not be powerful enough to match the existing light that is present in the scene. Your subject may still be rendered too dark.
One way to cut down on the amount of exposure from the existing light is to increase the shutter speed while keeping the same aperture. This will darken the elements of your scene that are being illuminated by the existing light but not affect the parts of the image that are being lit by your speedlite.
You may need to use a fast shutter speed to freeze the action and still want to add some fill flash to fill in the shadows.
Last week I need to do some team photos of a local baseball league in the middle of the day in the middle of a field. I got lucky because it was overcast and I did not have to worry about my subjects squinting from bright harsh sunlight.
I wanted the background to be somewhat out of focus so I chose f 5. That gave me a shutter speed of 1/250 second. A normal sync speed of 1/60 second would required an f-stop setting of about f10. This would have rendered the background a little bit too sharp and focused.
If you don't use a high sync mode and your shutter speed is too fast for the flash to sync with your camera only part of your frame will receive exposure from the flash fires.
Photographing a white wall in a dark environment would result in a photograph like this:
Watch less TV.
Shoot more photos.