I have observed many yoga students who hyperextend their necks in poses like Cobra. In the case of backbends, many students have the habit of over-arching from the neck. They compress the backs of their necks, and often arch too much from the back of the skull (the occiput), at the atlas-occipital joint. (The atlas is another name for the first cervical vertebra.) This habit is common in those with tight thoracic spines. When the thoracic spine is stubborn, people sometimes overcompensate by overarching the more flexible cervical and/or the lumbar spines. In other words, when one link in the chain is tight, people tend to move more than they should from the links above and below it.
When instructing backbends, I encourage students to try to keep the back of the neck long. Rather than looking up in a pose like Cobra, I encourage those with the habit of neck hyperextension to keep their gaze forward, which tends to keep them from tipping the head back too much. It’s also useful to think of originating the movement in your neck from the middle of the thoracic spine and the lower cervical vertebrae (where the neck attaches to the back).
In twists, try not to lead with your head. In other words, the turn should come from the vertebrae all along your spine, with no twist whatsoever from the atlas-occipital joint. One instruction I give if students feel any tension in the neck is to turn the head ever so slightly (say 1 millimeter) in the opposite direction of the twist. What this accomplishes is to stop people from trying to twist the skull on C1, a motion those joints are not meant to do. Even more conservative, is to not let the chin turn any more than the chest, in other words the nose and chest point in the same direction.
Beyond lessening the theoretical risk of a vertebral artery stroke, all the above advice will also tend to help avert yoga’s contributing to such musculoskeletal problems of the neck as arthritis and overstretching of spinal ligaments.
Excerpt from Yoga for Healthy Aging. The entire article can be found by clicking "here".