Final answer:
The increased movement between the C1 and C2 vertebrae is enabled by the atlantoaxial joint, which is a pivot joint allowing for the rotation of the head.
Step-by-step explanation:
What allows for increased movement between the C1 and C2 vertebrae is the presence of a specialized joint known as the atlantoaxial joint. This joint is different from the other intervertebral joints in that it is a pivot joint which specifically provides for rotation of the head. Unlike the zygapophysial joints (facet joints) in the rest of the vertebral column that allow for limited motion, the unique structure of the atlantoaxial joint allows for considerable rotational movement. This joint, in combination with the atlanto-occipital joint that allows for flexion and extension, facilitates the wide range of motion we experience in our heads.