Canon Speedlite 600 EX-RT User Manual (en, zh)

By pointing the flash head toward a wall or ceiling, the flash will bounce 
off the surface before illuminating the subject. This can soften shadows 
behind the subject for a more natural-looking shot. This is called bounce 
Set the Bounce Direction
You can turn (bounce) the flash head while pressing the <z> 
button as shown. During bounce shooting, the flash icon on the LCD 
panel changes to <m>.
When the flash head is turned while the flash coverage is set to <L> 
(automatic) (p.29), the flash coverage is fixed at 50 mm and <---> is 
displayed on the LCD panel.
You can also set the flash coverage manually (p.29).
If the wall or ceiling is too far away, the bounced flash might be too weak 
and result in underexposure.
If the picture appears dark or the flash exposure confirmation lamp does 
not light, use a larger aperture opening (smaller f/number) and try again. 
You can also increase the ISO speed when using a digital camera.
The wall or ceiling should be plain white for high reflectance. If the 
bounce surface is not white, a color cast may result in the picture.