Did you know there are 26 muscles that control the shoulder?! However ONLY 4 are uni-articulating (only crossing the gleno-humeral joint) and are considered giving the Gleno-humeral joint dynamic joint stability. These are called the Rotator Cuff muscles which include the Subscapularis, Supraspinatus, Infraspinatus, and Teres minor.
As the deltoids pull the head of the humerus upwardly, without a strong unopposed rotator cuff, one would have impingement. Meaning that the rotator cuff muscles pull the head of the Humerus down as well as compress it within the Glenoid cavity to both increase stability as well as reduce impingment.
The scaption plane is elevating the arm 30 degrees in front of Abducting it straight to the side, this is the ideal plane to train the rotator cuff because it put’s your gleno-humeral joint in line with the scapula as well as giving ideal muscle length for the shoulder stabilizers/rotator cuff muscles to activate in.
I personally warm up with this exercise before any Upper Extrimity exercise because it primes my nervous system to activate these muscle in order to stabilize the shoulder, which may PREVENT shoulder injury from occurring.