Passive forearm supination (Forearm pronators stretch, sitting)

Sit upright in a chair.
Bend your elbow and tuck it in to your side keeping your wrist and fingers relaxed.
Hold your forearm with your opposite hand, just below your wrist.
Use your opposite hand to turn your forearm so that your palm turns towards the ceiling.
Hold this position in a stretch before relaxing the arm. 

Ensure you do not lean your body with this movement.