This movement can be done with a rope, stick or sheet. you may not be able to go all the way back when you start and thats ok. Stay on the sweet side of the stretch. The front and back stretch is the basic move. Exploring the sides is a more advanced movement.
Very often the shoulder gets locked down in this armpit area. teres Major, teres minor, subscapularis, infraspinatus and serratus anterior often get really locked up. This movement should be done in conjunction with the above shoulder mobility stretch.
Long Chain Shoulder Mobility
Note: We are stretching the shoulder in relation to the rest of the body here. Think if the shoulder joint itself as just a waypoint on a longer journey.