Sensitivity slider allows you to set the top speed of the mouse cursor. Leftmost position is the slowest, rightmost the fastest. Please note, that this setting doesn't change the sensitivity curve visualization (the view is normalized to the sensitivity).
Deadzone slider specifies percentage of the full movement range (up to 50%) that is ignored and doesn't cause any cursor movement. It allows for neck muscle relaxation as it doesn't require you to hold your head exactly centered to hold the cursor steady.
Curvature slider is used to modify the feel of the tracker; when moved to the right off the center position, the tracking is more precise around the center; when moved to the left, the speed rises more abruptly allowing for faster, yet less precise cursor control.
Step only checkbox disables the Curvature slider and causes the cursor either stand still (when inside the deadzone) or running the full speed in the direction of head movement.
Apply changes button allows you to check the new settings; when the result is unsatisfactory, either click the Revert changes button or wait for the timeout to get back to the previous state. If you like the settings, make sure you click the Keep button!
Calibrate button commences the calibration process. The first stage is the recentering - center your sight and wait for the timeout; the calibration itself is started next - before the time is up, move your head to the left/right/top/bottom extremes. It allows the software to optimize the tracking to the range of motion of your head.
Recenter button allows only to center the view without going through the full blown calibration.