The Smart screen feature
The Smart screen feature keeps the phone from activating screen time-out when it detects your eyes looking at the screen. When you turn this feature on, you can set a shorter screen time-out to conserve your battery power, but still allow you to view a screen for a longer period of time without having to touch it to keep it activated.
To turn this feature on, from the Home screen, touch and hold the Recent Apps Key 
(in the Touch Keys bar) > Settings > Display > checkmark the Smart
screen checkbox.
The following situations may prevent these features from functioning properly.
When the phone is in the dark or against bright light.
When the camera lens can’t detect the location of your eyes.
When the front camera is used at the same time with other apps.