The brachioradialis reflex (also known as supinator reflex [1])is observed during a neurological exam by striking the brachioradialis tendon (at its insertion at the base of the wrist into the radial styloid process (radial side of wrist around 4 inches proximal to base of thumb)) directly with a reflex hammer when the patient's arm is relaxing. This reflex is carried by the radial nerve (spinal level: C6,C7)
The reflex should cause slight wrist extension and/or radial deviation, supination and slight elbow flexion.
References
Brachioradialis reflex Wikipedia(Text) CC BY-SA