Understanding the basics of the "Traps' helps you create awareness in how you stand, not only in your Yoga practise but generally speaking. Look at the image below and visualise the middle section of the Trapezius contracting to bring the shoulder blades closer together. This awareness will help you find your authentic posture where your bones are properly aligned and your muscles, joints and ligaments can work efficiently. Good posture and good health go hand in hand. As Loretta Young once said, "Unless some misfortune has made it impossible, everyone can have good posture".
The Trapezius or “traps”, run from the base of the skull and the spine out to the shoulder blades, covering most of your middle and upper back.
Although the upper and lower parts of the trapezius help in bringing the shoulder blades closer together, it is mainly the middle section of the trapezius (whose fibers run horizontally from the upper and middle part of the spine to the inner edge of the shoulder blade) which does much of the work of pulling the shoulder blades towards the spine.