The Smart screen and Smart video features
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 Front 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 cannot detect the location of your eyes.
When the front camera is used at the same time with other applications.