The fencing response is an involuntary, abnormal posture—one arm extended, the other flexed—that can occur after a traumatic brain injury, mimicking the stance in fencing, and is named after the asymmetrical tonic neck reflex (ATNR) seen in infants after head turns. It's a key indicator of brainstem involvement after impacts in sports like football, boxing, and rugby, signaling a neurological disturbance, though recent studies suggest it doesn't necessarily predict worse concussion recovery time, despite its unsettling appearance.