By having the PRE-AF on, the camera continuously focuses. As long as the power switch is on, it will focus. So you do not need to press the shutter button halfway down for focusing.
The PRE-AF speeds up the lag in the focusing of the time it takes to half-press the shutter button.
Menu > AF/MF setting tab > PRE-AF > ON | OFF
The PRE-AF uses about 40% more power than not having …