Two muscles in the forearm, the pronator teres and pronator quadratus, work together to achieve pronation by pulling on the radius bone of the forearm. Theradius is specially designed to rotate at the elbow and wrist joints around the other forearm bone, the ulna.
If you work with your hands a lot chances are you have ticked off your pronator teres before. This is not a large muscle but it has a large belly that you can easily palpate. To activate the pronator teres, extend your are out with your palm facing the sky, then flip your palm so it is facing the floor, that is promotion. The pronator teres is one of the easier muscles to self treat.
Please refer to the about video so stretches.