To perform the test, the examiner grasps the wrist with their thumb over the scaphoid tubercle (volar aspect of the palm) in order to prevent the scaphoid from moving into its more vertically oriented position in ulnar deviation.
If the scapholunate ligament is disrupted, the scaphoid will subluxate over the dorsal lip of the distal radius.
Original Description by Watson:"The patient is approached by the examiner as if to engage in arm wrestling, face to face across a table with diagonally opposed hands raised (right to right or left to left) and elbows resting on the surface in between.
With the patient's forearm slightly pronated, the examiner grasps the wrist from the radial side, placing his thumb on the scaphoid tuberosity (as if pushing a button to open a car door) and wrapping his fingers around the distal radius.
The examiners thumb pressure opposes this normal rotation, causing the scaphoid to shift in relation to the other bones of the carpus.