The S2 Eye Tracker provides the flexibility for subjects to move their heads a reasonable amount; to a level that easily goes past what is normal behavior when looking at a screen. As an example, the video below shows a sample eye tracking study with a subject that’s shaking his head while maintaining his gaze at a single point.
On the right hand side you see a recording of the subject. You can see the subject shaking his head quite significantly, while his gaze is fixated (for the most part) on a single item on the desktop. The movement is represented by the red lines, and the fixation (how long the subject looks at a single point) is represented by the red circles.