Backlight setting "at deepth".

Going from memory, the backlight turns off/on at approximately 20 feet (6 meters) when set to "At Depth". This ended up being fairly inconvenient when I forgot to change the setting before a night dive, as the display wanted to stay dark during the safety stop, so I stick with "Always On" these days.

The aforementioned night dive was was several firmware updates ago, however, so it's possible that he depth threshold has been adjusted since then.