The Smart screen and Smart video features keep the device from
activating screen timeout when it detects your eyes looking at the
screen. When you turn these features on, you can set a shorter screen
timeout setting 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 these features on, from the Home screen, tap the Menu key 
(in the Touch Keys Bar) > Settings > Display > checkmark the Smart
screen checkbox and/or the Smart video checkbox.
The following situations may prevent these features from functioning properly:
When the device is in the dark or against bright light.
When the front camera lens cannot detect the location of your eyes.
When the front camera is used at the same time with other apps.