Found in the Settings Section: Low power mode will limit features to use less power. As a result the video preview will be turned off as it uses CPU intensely.
Features that will still work on low power mode include:
Camera: Focus Lock
Audio: Mute, Gain
Effects: All will be turned off
Manual: Brightness, Exposure, Saturation, White balance
Setting: Startup, Appearance(Only Anti-Banding)