The Spencer technique, in which a clinician guides the shoulder joint through its full range of motion (ROM), may prevent injury in baseball pitchers, according to research in the Journal of the American Osteopathic Association[1].
Researchers evaluated the effects of the Spencer technique on pitchers from Seton Hill University's men's baseball team. They found a single administration of the technique immediately restored internal rotation of the players' shoulder back toward baseline.

Video courtesy of Didactics Online.

"We know repeated overhead throwing alters range of motion in the shoulder, which can hinder performance and increase susceptibility to injury," said Amber Eade, PhD, lead researcher on this study. "Physical therapists and trainers have been using the Spencer technique to address this problem; however, there has been no research to support that approach until now."
The researchers measured players' ROM to establish a baseline, then came back a week later to measure again. In that short window, a significant 14 percent reduction of internal rotation in the shoulder joint had already occurred as a result of training. Researchers then administered the Spencer technique and reevaluated, finding the players' internal rotation was restored 85 percent back toward the first week's measurements.
Effect of treatment on range of motion in collegiate baseball pitchers (N=15). Mean (SE) degrees of movement from week-2 pretreatment and posttreatment range of motion measurements are illustrated for (A) the Spencer group and (B) the sham therapy group. Significant changes in range of motion after treatment were noted for internal rotation and abduction of the Spencer group only. a P<.05.


"Considering that study participants were college-level players and the vast majority had been pitching several years, it was surprising to see the effects a week of playing had on their range of motion," said Stacey England, DO, the osteopathic physician overseeing the study. 
"Osteopathic medicine is focused on prevention, so it was equally encouraging to see the effect of the Spencer technique[2]. This is a great first step in determining the full potential of this technique for baseball players and whether more frequent administration can reduce rates of shoulder injury in follow-up studies."
Considering the study's extremely small group of participants, it's difficult to envision how researchers can say this. In addition, despite this technique being around for over 100 years – it was developed by Charles H. Spencer, D.O. in 1915 – it mostly stands apart from widespread acceptance in baseball and training circles as a preventative measure.
Moreover, in consulting the website of Dr. James Andrews, the well-known sports orthopedic surgeon widely accepted to be the foremost authority on shoulder and elbow treatments, a search of the Spencer technique did not produce even a single result.
The study's positive results appear to be, in a word, anecdotal. All this is not to say that making the shoulder joint more supple isn't worthwhile. Just that this very limited study doesn't make its case terribly well. 
1. Curcio, Janine E., Matthew J. Grana, Stacey England, Paige M. Banyas, Benjamin D. Palmer, Arielle E. Placke, William A. Rieck, and Amber M. Eade. “Use of the Spencer Technique on Collegiate Baseball Players: Effect on Physical Performance and Self-Report Measures.” The Journal of the American Osteopathic Association 117, no. 3 (March 1, 2017): 166–75. doi:10.7556/jaoa.2017.031.
2. Patriquin, D. A. “The Evolution of Osteopathic Manipulative Technique: The Spencer Technique.” The Journal of the American Osteopathic Association 92, no. 9 (September 1992): 1134–36, 1139–46.