This description of the Horizontal Gaze Nystagmus test posted by Virginia DUI Lawyer.

•  Nystagmus simply means an involuntary jerking of the eyeballs. This jerking becomes more distinct and obvious as the amount of alcohol increases in the subject's bloodstream. Alcohol will enhance the nystagmus in order to make it more readily noticeable. If under the influence of an alcoholic beverage, the subject is powerless to stop or control it. For the most part, it is unconscious. The subject is unaware that the nystagmus is happening, but some individuals claim they can feel the jerking occur. 

•  The police officer is trained in the Standardized Field Sobriety Testing Course to ideally utilize a penlight at roadside to assist in the administration and observation of nystagmus in the suspect's eyes. This stimulus can be as simple as your index finger or pen. 

•  Use of a flashlight is not recommended due to the probability of casting of shadows which will make it more difficult to make a fair assessment of the presence of nystagmus. Depending on the location of the flashlight, it can also cause an artificial blinding and/or cast shadows which make the nystagmus more difficult to observe.