The Inverted W is a term that is used to refer to one member of a family of arm actions that you see in baseball pitchers. I first discussed the Inverted W, and the other related Inverted Arm Actions, in a series of articles on my public site...
The Inverted W and the related arm actions are widely taught because they will give a pitcher a quick velocity boost. However, I have found that they will also significantly increase a pitcher's risk of experience an elbow and/or shoulder injury.
The idea that the Inverted W is problematic is fairly controversial, in large part because it hasn't been backed up by significant research. However, that research has been done and it backs up the things that I have said about the Inverted W and the other Inverted Arm Actions.
In particular, Dr. Weimi Douoguih's study of the Inverted W directly addresses the questions of whether the Inverted W is problematic and why.
The Inverted W: Prevention & Management is currently a work in progress. However I have made available my in-progress work so that my clients can get a jump start on preventing and managing the Inverted W in their pitchers.