A more thorough explanation:

The horizontal-gaze nystagmus test is a field-sobriety test used to determine if a person is intoxicated. During the test, the suspect is asked to focus on an object, such as a pencil, and track its movement from side to side using only their eyes. If the eyes jerk or twitch while tracking the object, it is an indication of intoxication. The test is abbreviated as HGN test.

When a police officer suspects a driver of being under the influence of alcohol, they may administer a horizontal-gaze nystagmus test. The officer will ask the driver to follow a pen or finger with their eyes as it moves from side to side. If the driver's eyes jerk or twitch while following the object, it is a sign of intoxication.

Another example is when a person is suspected of being under the influence of drugs. The officer may use the HGN test to determine if the person is impaired. If the person's eyes jerk or twitch while following the object, it is an indication of drug use.

These examples illustrate how the horizontal-gaze nystagmus test is used to determine if a person is intoxicated or impaired. It is a simple test that can be administered quickly and easily in the field.