What does the ""Set the screen brightness for best energy efficiency" actually do?

This is designed to optimize your display brightness to reduce power consumption and extend battery life on laptops and tablets.

When this option is enabled, this setting allows Windows to automatically adjust the screen brightness based on ambient lighting conditions. This is achieved through sensors that detect the surrounding light and adjust the brightness accordingly to ensure optimal visibility while conserving energy

This feature is often integrated into Windows power plans. When you select a power plan optimized for battery life, such as "Power Saver" or "Battery Saver" mode, the screen brightness will be automatically adjusted to lower levels to reduce battery drain.

Changes screen brightness. Power and battery page can set your display to 60Hz if it's a high refresh rate one.

That 'Set the screen brightness for best energy efficiency' is just a recommendation that you turn down your brightness when your laptop is on battery power, it is not an actual setting and if you click that it just takes you to the Display settings for you to make changes to your brightness.

If you find that this feature isn't working well for you, you can usually toggle it off or adjust its settings to better suit your preferences.

The next thing to check is the control panel for your video card from the right click menu on your Desktop, to see if there is any setting there causing this, most of them do have separate power settings too.