PRO/PROX devices utilize Inertial Measurement Units (IMUs) to accurately monitor the participant's head rotation and movement. When you attach the amplifier to the EEG cap, its position allows it to move in sync with the participant's head, facilitating precise tracking.
If you're using Smartfones for example, the amplifier is outside of the headset so it can't move with the head. Same goes for sleep studies, where the amplifier would be on a chest belt.
In mbtStreamer/SmartingProApp software you need to enable 3D Motion in Settings. This will turn on quaternion streaming:
Once you turn on the 3D Head view, you'll get a notification to calibrate the amplifier's motion sensors! To do this, you need to shake the amplifier until the notification disappears.