The glabellar tap is a primitive reflex where the eyes shut if an individual is tapped lightly between the eyebrows. This reflex may normally be overcome rapidly - i.e. the individual soon fails to blink, usually less than five taps.
In a patient with frontal release signs the reflex cannot be overcome, and they continue to blink for as long as the examiner cares to keep tapping. A similar response is seen with late parkinsonism.