The carotid is more easily seen in this image than the vertebral artery.

Vertebral Artery

Vertebral Artery

From the eMedicine site:

The typical presentation of VAD is a young person with severe occipital headache and posterior nuchal pain following a recent, relatively minor, head or neck injury. The trauma is generally from a trivial mechanism but is associated with some degree of cervical distortion.

Focal neurologic signs attributable to ischemia of the brain stem or cerebellum ultimately develop in 85% of patients; however, a latent period as long as 3 days between the onset of pain and the development of CNS sequelae is not uncommon. Delays of weeks and years also have been reported. Many patients present only at the onset of neurologic symptoms.

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The major causes of stroke in young adults and children differ from those in older people. Dissection of the internal carotid and vertebral arteries in the neck accounts for about a fifth of strokes in the young compared with about 2.5% in older patients.

…The incidence of arterial dissection is increased in patients with fibromuscular dysplasia, migraine, or hypertension; in smokers; and in those taking oral contraceptives. It is commonly associated with trauma or manipulation to the neck.

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From the above it seems clear that care should be taken by to avoid rapid and extreme flexion, extension, and rotation of one’s partner’s head. Percussion of the neck should likewise be avoided. Those techniques that require such movements should be modified by limiting the velocity and degree of manipulation, and/or contact. Children’s necks are especially vulnerable and therefore these techniques are not appropriately practiced on them. Newer students who have not yet demonstrated their ability to use discretion in their application of force in partner work should not be encouraged or allowed to practice these kinds of techniques.

Instructors should carefully observe students when they are engaged in practicing encounters in which neck manipiulation is required, and should take seriously any complaints of headache, nausea, changes in vision, or other related symptoms.