Canon 600EXII-RT Instruction Manual page 23

Turning on the Power
The flash settings will remain in effect even after the power is turned off.
To retain the settings when replacing the batteries, replace the batteries
after turning off the power switch.
You can fire a test flash while the power switch is set to <a>. Also,
when a button or dial is operated, the LCD panel illuminates.
You can set a beep to sound when the Speedlite is fully charged (C.Fn-
20/p.116).
Auto power off can be disabled (C.Fn-01/p.113).
You can change the setting of the LCD panel illumination (C.Fn-22/
p.117).
You can change the color of the LCD panel illumination (P.Fn-02 to 04/
p.118).
You can disable Quick flash (P.Fn-05/p.119).
When P.Fn-06-1 is set (p.119) during linked shooting (p. ), you can fire
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the Quick flash.
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