When we bear weight through the arms, the shoulder blades are meant to stabilize on the back.
If the muscles that surround the shoulder blades are balanced in tone, they all do their thing and the shoulder blade remains relatively where they began.
But very few people have balanced tone in the muscles of the arms and shoulder blades.
When the arm lifts out to the side or above the head, the shoulder blades are meant to move with the arm.
Unfortunately, due to assorted tightnesses and imbalances, the shoulder blades are often unable to work correctly.
The host of muscles involved in all of these actions includes the rhomboids, serratus anterior, lower trapezius, pectoralis minor, teres major, and the four muscles of the rotator cuff, etc.
They all perform the dance meant to move yet keep the shoulder at home.
The exercise in the video (my darling wife’s first appearance in one of my videos) gets heavy, deep, and real.
There is a lot of work involved on the part of the assister and a lot of intense sensation on the part of the receiver.
It is often the case that muscles that have rarely been used are being profoundly stretched and opened.
Any muscles connecting the arm to the shoulder blades are in play.
Whether the action is pulling the arm away or towards the body the muscle will be affected.
Teres major and minor, the deltoids, the supraspinatus, the biceps brachii are all involved.
The power of this exercise is to open and create a great deal more range of motion than before.
I have always hesitated to share it because it can be very intense and it takes a good amount of strength to pull it off correctly if the muscles in question are tight.
If those muscles are tight the shoulder blades will not want to stay in the position the assister is putting them in and it can be exhausting trying to keep them there.
I often use the assist for comic effect in workshops because the look on the face of the person stretching can be extreme.
Good luck.