The 90/90 Shoulder Rotation is an effective exercise for restoring and gaining mobility in the shoulder girdle. It especially targets the external rotator cuff (infraspinatus and teres minor) and the shoulder capsule. It can be done with a stick/dowel or a loaded bar. The quality of the movement should precede the load, and you should never train through pain.
(See the Quality Before Quantity Principle)
This movement helps to strengthen and lengthen the external rotators of the humerus and it promotes a healthy range-of-motion in the shoulder capsule. This equates to a properly functioning shoulder and greater stability in this extremely mobile area of the body.
I have found this exercise to be most beneficial with my clients when using a 4 seconds up and 4 seconds down tempo. Other slow and controlled tempos can work well, too. 3 sets of 5 reps is a great place to start. With perfect technique, one can gradually move up to 3x15. Other sets and reps ranges could be used for different contexts.
Pay attention to keeping the scapula depressed and slightly retracted; in the beginning of the video you can see how I intentionally elevate and then depress the scapula to "anchor" them down (shoulders away from ears). The elbows and arms are at 90 degrees. Try not to elevate (hike up) the scapula as you lower the hands in internal rotation. This helps to develop axial rotation of the glenohumeral joint without the scapula coupling along. If you can't lower the hands far down to touch the bar to the front of the body, simply go down as far as you currently can without hiking up the scapula. When learning this exercises, it can be helpful to use a mirror.
Practicing this exercise regularly with a stick is great for mobilizing the shoulder joint. If you are using this exercise for holistic strength training, its beneficial to be patient and start with light weights.
Overall, this is a valuable exercise for strong, mobile and healthy shoulders.