Out of all the Standard Field Sobriety Tests, the HGN is the easiest to fail. No one, but the officer, can see the results. In fact, there is never a video of the person's performance on the HGN. Generally, if the officer was quick enough to turn on the dash cam, then there will be a recording o the officer administering the test. However, the camera is too far away to actually see the results.
An officer will stand before a suspect with a pen, pen light, or pencil in one hand and a flashlight in another hand. The officer will ask the person if they are wearing contact lenses or have any eye problems. According to NHTSA, there is no variation in results if the officer administers the test to someone who is wearing contact lenses.
The officer then instructs the suspect to follow the pen, pen light or pencil that he is holding in front of the suspect's face, as he moves to the left or right. The person is supposed to keep his or her head still. Often times, the suspect will not and the officer will enjoy telling the jury that the defendant was not listening to instructions.