Lock instantly with power key
When enabled, the setting of the Lock phone after / Lock automatically option will be ignored when the screen is turned off using the power button on the device, causing the device to lock instantly.
The Lock phone after / Lock automatically option will still be honored when the screen turns off by itself due to a timeout.
In effect this setting controls whether the Lock phone after / Lock automatically option always applies when the screen turns off, or only applies if the screen turns off by itself.
This option is only available on some devices.
When you have not set a delay in Lock phone after / Lock automatically, this setting has no effect and your device will always lock as soon as the screen turns off.
Note: some carriers have reportedly removed this setting from carrier-branded phones they distribute.