The posterior rotator cuff consist of three of the four rotator cuff muscles; Infraspinatus, Supraspinatus & Teres Minor (which are External Rotators of the Shoulder). Now that you have learned how to give this area some flexibility (from the previous episode) here is a great way to strengthen these muscles eccentrically.
WHY eccentrically you ask?
Regarding rotator cuff strength, it is generally recognized that overhead athletes often exhibit sport-specific adaptations leading to a relative decrease in the strength of the external rotators, and thus muscular imbalance in the rotator cuff. With overhead athletes the recent focus has shifted from isometric or concentric to ECCENTRIC muscle strengthening of the rotator cuff (particularly the External rotators). Reason being that these muscles are used as a decelerator during powerful throwing, serving, or smashing movements.
More information @ “Prevention of shoulder injuries in overhead athletes: a science-based approach” by: Ann Cools et al.