If you tap the settings button (top right), you are presented with the settings options.
The "Orientation" options affects the way the viewport locks in locked mode.
- Auto: Horizontal or vertical orientation will be chosen depending on the device physical orientation. If you start recording, the orientation automatically selected will not change.
- Horizontal: Use this, if you intend to hold your device horizontally.
- Vertical: Use this, if you intend to hold your device vertically.