The Smart screen feature
The Smart screen feature keeps the device from activating screen timeout when it detects your eyes looking at the screen. When you turn this feature 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 Smart Screen on, from the Home screen, tap (or press) the Menu Key 
> Settings > Display > checkmark the Smart screen checkbox.
The following situations may prevent the Smart screen feature 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.