Posterior capsular tightness pushes the humeral head forward out of the glenohumeral joint resulting in impingement between the rotator cuff and the anterior acromion.
What's the best way to stretch the posterior capsule? That's something physical therapists are studying. Patients with posterior shoulder tightness show up most often in a sports clinic. Athletes who can't fully rotate the shoulder inwardly can be at a disadvantage in their sports performance. This is especially true for athletes involved in sports that require overhead throwing.
In this study, a group of therapists compare which works better: just the stretching technique or stretching combined with joint mobilization. They didn't include a control group (people with this problem who don't receive any treatment) or a group who just got mobilization. All the subjects were healthy college-aged adults. They each had at least a 10-degree difference in shoulder internal rotation from one side to the other. That's why they were chosen for the study. None of the participants had any actual pain or other symptoms.