From BR Bullpen
Mechanics is the ability that a pitcher has to reproduce his delivery pitch after pitch. The term comes from a comparison with a machine which is able to execute the exact same movement time after time.
Mechanics are an important issue in evaluating a pitcher's effectiveness and potential. Good mechanics mean a pitching motion without unnecessary gestures, that maximizes velocity and control, that does not place undue stress on any part of the body, and that can be reproduced consistently. Poor mechanics can refer to a motion where velocity is generated solely from the arms (and not from the legs), where the release point is inconsistent, etc. Poor mechanics are usually seen as an indicator of future problems with wildness or injury.
There is no real consensus as to what are good or perfect mechanics, and often the statement from a scout that a pitcher has good mechanics simply means that he does nothing unusual when he pitches. Generally, Tom Seaver and Nolan Ryan, who were both able to generate tremendous power through the use of their legs and pitched injury-free well into their 40s, are held as examples of outstanding mechanics. However, pitchers with unusual motions have also had long and successful careers. Examples include Luis Tiant and Gene Garber, who both turned around almost completely toward second base in the course of their delivery, or Juan Marichal, who had a very high leg kick. Conversely, Mark Prior was widely said to have "perfect mechanics" when he was pitching in college, but his career was cut short by the sort of arm injuries typical of pitchers who have poor mechanics.
Poor mechanics are usually easier to diagnose, although the question is then whether they should be fixed, or whether a pitcher who is successful in spite of them should be allowed to continue with what works for him. In the case of poor mechanics resulting in reduced velocity, poor control or repeated injuries, there is little hesitation for the pitching coach to attempt changes.
As a result, the subject of mechanics remains a controversial one, with widely divergent views about whether a given pitcher has good, bad or simply unusual mechanics. The issue, which was traditionally the purview of scouts and pitching coaches, has become increasingly discussed in public since the beginning of the 2000s, with outsiders such as Will Carroll of Baseball Prospectus and former pitchers such as Carlos Gomez and Mike Marshall writing prominently on the subject in various fora.