Mariners' closer J.J. Putz is mentioned in Will Carroll's latest Under the Knife column (subscription required) at Baseball Prospectus. Since the article requires a paid subscription (which is worth every penny), I won't talk at length about what he says, but I did want to mention this part of what Will said:

I'm a bit hesitant to give all the blame/credit to this since Putz "slammed closed" his elbow and that hard hyperextension is not often the result of a mechanical change. The difference is so noticeable that pitchers normally quickly alter things unless they simply cannot make the necessary adjustments. Some have been confused by the "slammed closed" designation I used previously. It's simply taking the elbow joint to its fullest extension, with the arm straight. The "closed" part refers to the bones and their spacing, not the joint's extension. (ed: emphasis mine)

I like Will's term of "slammed closed," which is shorthand for "slammed his olecranon process into its fossa." When you supinate the release of your pitches (note the wrist position of Putz in the above photograph), that's exactly what you do. It typically manifests itself in the lengthening of the ulna bone and the reduction of flexion about the elbow, both of which are irreparable damage and not correctable by surgery. However, it also irritates the ulna nerve and can rupture the ulnar collateral ligament, which is why Putz was so afraid of needing Tommy John surgery after feeling shooting pain and numbness in the fingers (forearm tightness and pain are the other two common symptoms of a ruptured/torn ulnar collateral ligament). Read this great excerpt (free) from Biomechanics in Sport available from Google Books to learn more.