Pec minor originates from the top of the scapular at the coracoid process and inserts into ribs 3, 4, and 5. When this muscle is short it can contribute to rounded shoulders and shoulder issues such as impingement.
This stretch is aimed at stretching out the hip flexor muscles (predominately psoas and iliacus). These muscles can become tight with prolonged sitting and can be a factor in low back stiffness.
This exercise works many muscles of the back as well as the biceps, abdominals, and legs.
The gluteus medius muscle is an important muscle of the hip and is hugely important for knee health, hip health, and back health. This muscle can be weak (and is often weak) in individuals who have issues in any of these three joints. The exercise in the video is a good beginning movement for strengthening the gluteus medius muscle.