A more thorough explanation:

HGN Test

The HGN test, or Horizontal Gaze Nystagmus test, is a field sobriety test used by law enforcement officers to determine if a person is under the influence of alcohol or drugs. The test involves the officer observing the person's eyes as they follow a moving object, such as a pen or flashlight, to detect any involuntary jerking of the eyes that may indicate impairment.

During a traffic stop, a police officer suspects that the driver may be under the influence of alcohol. The officer asks the driver to step out of the car and perform a series of field sobriety tests, including the HGN test. The officer holds a pen or flashlight about 12-15 inches away from the driver's face and moves it from side to side while observing the driver's eyes for any signs of nystagmus.

If the driver's eyes exhibit nystagmus, it may indicate that they are under the influence of alcohol or drugs. The officer may then proceed with further testing, such as a breathalyzer or blood test, to confirm their suspicions.

This example illustrates how the HGN test is used by law enforcement officers to detect impairment in drivers suspected of driving under the influence. By observing the driver's eyes for involuntary jerking, the officer can determine if further testing is necessary to confirm impairment.