Excellent research from last year's American Society of Biomechanics meeting from a few Japanese researchers, "A MECHANICAL CAUSE OF BODY ROTATION ABOUT THE VERTICAL AXIS IN BASEBALL BATTING". The authors concluding, "Such a rotation of the entire body could only be generated by the external forces acting eccentric to the center of mass and by the free moments acting on the body. The results clearly indicate that the rotation of the body during baseball batting is generated primarily by the moment of the ground reaction forces acting on the legs around the center of mass".
This drives home what we high school A.T.'s have always known, that is the transverse plane (rotation) is primarily driven by momentum, whether it's swinging a bat or throwing a baseball.
Yet, at last year's combined sections meeting of the APTA, a statement was made that "what is causing the torso to rotate is the muscles, so we have to work on the obliques, the core itself, and hip stability".
Now go back & read Vern's "Rotation is bad?" post from December 13 and decide for yourself who is right.