Formally a sign in neurology, the Babinski sign, or Babinski response, is one of several abnormal pathological reflexes that occur in a patient with Central Nervous System damage. It is considered present when extension of the big toe and fanning of the other toes occurs while performing the plantar reflex during physical examination of a patient.
The plantar reflex is performed by stroking the outside of the sole of the foot and across the base of the toes to the big toe and observing the involuntary response of the patient's foot. A normal foot response would be for the big toe to flex and the other toes to fold toward the sole of the foot and is noted as an absent Babinski sign.
Testing to see if a Babinski sign is present is a basic part of a physical examination and a screening tool to determine if there is nerve damage occurring from the brain through to the spinal cord along the corticospinal tract. It is simple, quick, and reasonably sensitive although nonspecific for upper motor neuron disorders.
The examiner applies pressure along the outside of the sole of each foot then moves across the base of the toes to the big toe, using increasingly firm touch and sharp probing. Initially, lightly stroke with a finger, then with a key or other blunt object, then with slightly pointed implement such as a wooden applicator stick broken in half. The normal response is for the big toe to turn downward and the toes to curl. If the Babinski sign is present, the big toe turns upward and the toes may fan. Lack of movement of the toe does not have strong significance.
The exact mechanism is not completely understood, but electromyographic studies suggest that skin areas, in the distribution of spinal nerves, appear to have specific pain reflexes, primarily in the spinal cord that are affected by the brain. "The area of skin from which the reflex can be obtained is known as the receptive field of the reflex. To be more specific, a noxious stimulus to the sole of the foot, which is the receptive field, causes immediate flexion of the toes, ankle, knee, and hip joints with attendant withdrawal of the foot from the stimulus...
The plantar reflex contrasts with the great toe reflex: "Stimulation of the ball of the toe, which is the receptive field, causes extension (dorsiflexion) of the toe with flexion at ankle, knee, and hip joints. The differences between these two reflexes are in the receptive fields and the fact that the great toe is flexed in one and extended in the other. The reason for the extension in the toe reflex is to remove the toe from the stimulus."
An abnormal plantar reflex produces extension of the great toe instead of the normal flexion response. 
"...The abnormal response reliably indicates metabolic or structural abnormality in the corticospinal system upstream from the segmental reflex. Thus the extensor reflex has been observed in structural lesions such as hemorrhage, brain and spinal cord tumors, and multiple sclerosis, and in abnormal metabolic states such as hypoglycemia, hypoxia, and anesthesia."
- H. Kenneth Walker, The Plantar Reflex, in Walker HK , Hall WD, Hurst JD, Clinical Methods: The History, Physical and Laboratory Examinations (Third Edition ed.), Butterworth