Actually in Anusara there is not a "neck loop". I am guessing that you are referring to "shoulder loop":

Shoulder loop appears in every pose. It is initiated by drawing back the upper palate toward the back of the neck. Then keeping the side body long with the head of the arm bones up and back, draw the lower tips of the shoulder blades down and into the back of the heart. On the front of the chest you will then feel the sternum lift to complete the loop.

Performed properly it will open the shoulders and upper chest and free up the neck.

If you are in the San Francisco area, there are many great Anusara teachers at Yoga Kula and Yoga Tree that can help you more.

Blessing, Scott

ps. the "Anusara" tribe is a little active than the "Anusara Yoga" tribe.