Which of the following clues relates to the ability to follow a stimulus smoothly?

The ability to follow a stimulus smoothly is a key component in assessing Horizontal Gaze Nystagmus (HGN). When a subject is asked to follow a stimulus, smooth tracking indicates that their vestibular system and eye coordination are functioning normally. If there is an inability to follow the stimulus smoothly, it suggests a disruption in these functions, which is one of the signs indicating possible impairment due to alcohol or drugs.

The other clues listed do pertain to aspects of HGN, such as onset of nystagmus before a specific angle and sustained nystagmus at maximum deviation. However, they measure different markers of impairment rather than directly indicating the capacity to follow a stimulus smoothly. The inability to follow indicates a clear dysfunction in the eye's tracking ability and is directly related to the assessment of HGN in relation to its significance in impairment detection.