Two Layers of Stretch
Depending on our purpose, we can analyze parts of the body into many different layers. For a discussion of joint movement, however, two are enough: the two layers of a joint are muscle and bone. Muscle includes muscle and tendon, while bone includes bone and the ligaments. Yogis should train themselves to feel the differences between the muscle and ligament sensations.
Drop your chin to your chest and relax. This is a passive, or Yin, stretch for the muscles and ligaments of the back of the neck. The muscles of the neck are on the left and right sides of the centerline. The ligaments we are concerned with are on the centerline. You can learn to feel the difference by comparing the sensations on each side of the neck with the sensations in the center.
Move the head to the right while it is still dropped forward. This movement stretches the muscles on the left side of the neck, making it easier to discriminate them. Moving the head to the left stretches the muscles on the right side of the neck. Bringing the head back to the center should help you distinguish sensations that are neither left nor right, but on the midline. These are the ligaments.
Muscular stretches feel sharper and are easily locatable. Ligament sensations are deeper, duller, and more attached to the bones. This is why Taoists use the expression "stretch your bones" to describe ligament stretches.
This simple exercise should be repeated many times. The distinctions may not be noticeable the first few times, but they become clear with experience. Notice that it is still possible to feel ligament stretches when the head is moved to the left and right. But by exaggerating the stretch on the muscles, it is easier to feel the difference between the two tissues.