A sustained natural apophyseal glide (SNAG) is a mobilization technique commonly used in the treatment of painful movement restrictions of the cervical spine. In the manual therapy literature, the biological basis and empirical efficacy of cervical SNAGs have received scant attention. In particular, an examination of their potential biological basis in order to stimulate informed discussion seems overdue. This paper discusses the likely biomechanical effects of both the accessory and physiological movement components of a unilateral cervical SNAG applied ipsilateral to the side of pain when treating painfully restricted cervical rotation. The use of flexion and extension SNAGS, and rotation SNAGS performed contralateral to the side of pain are not considered. Although a cervical SNAG may clinically be able to resolve painfully restricted cervical spine movement, it is difficult to explain biomechanically why a technique which first distracts (opens) and then compresses (closes) the zygapophyseal joint ipsilateral to the side of pain, and perhaps slightly distracts the uncovertebral cleft, would be superior to a technique which distracts the articular surfaces with both accessory and physiological movement components. Therefore, the reported clinical efficacy of cervical SNAGs cannot be explained purely on the basis of the resultant biomechanical effects in the cervical spine.