Knowledge of these bony articulations has important clinical applications:
Most cervical vertebrae have articular facets that permit extension-flexion and rotational movements; the joint between the atlas and axis however is specialized and permits only rotational movement; in accurate hanging, a large knot is placed posterior to the joint between the atlas and the axis and the weight of the body forcibly flexes the joint, driving the dens into the medulla, resulting in instantaneous death; in inaccurate hanging, the knot is placed behind other cervical joints, resulting in flexion of the neck, prolonged agony and death by strangulation; amateurs should not attempt hanging people.
Fortunately (or sadly, depending on one's perspective), there aren't too many hanging experts anymore.
Note: the death caused by brainstem impaction involves a cruciate ligament tear but is distinct from the Hangman's fracture. Further morbid reading is available from Wikipedia, including reports of fracture and even decapitation from hanging when the "long-drop" is too long.