The rotator cuff is a set of 4 muscles (and their associated tendons) that connect the arm bone (humorous) onto the shoulder blade. Their job is to articulate the arm in every direction. However they are not great at bearing the weight of the body on their own. The activations of I Y T and W are a POWERFUL practice to stabilize and integrate the rotator cuff, that way you can use the shoulder girdle to the fullest of its potential.