It converted brain waves into electromagnetic patterns and vice versa. These patterns could be relayed through a medical tricorder in order to control the autonomic functions of a person whose brain stem was damaged.
A neural pad was placed at the base of the skull, one on the patient and one on a volunteer. When connected, the brain of the volunteer could regulate the autonomic functions of the patient for a short period of time. A neural pad could be used to stabilize a patient when his or her injuries were too extensive to make use of a transporter. After this link, the volunteer needed to be given a full bioscan to make sure that there were no side effects, as the volunteer would noticeably be affected by this procedure. (TNG: "Transfigurations")