Lock instantly with power key
When Lock instantly with power key is enabled, your device will lock when you turn off its screen manually by briefly pressing the power key, regardless of the setting of the Lock phone after / Lock automatically option.
Otherwise, the delay specified in that option will apply even when turning the screen off manually.
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 automatically.
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 removed this setting from carrier-branded phones they distribute.