Final answer:
Side bending in the neutral lumbar spine produces a subtle movement of the sacrum known as countranutation or posterior rotation, which involves an upward and downward motion of either side of the sacrum.
Step-by-step explanation:
When the lumbar spine is neutral, side bending would result in a corresponding movement of the sacrum known as countranutation or posterior rotation. During lateral flexion, one side of the sacrum moves slightly upward and the other side moves slightly downward. This movement is subtle due to the stability provided by the sacroiliac joints and the supporting ligaments such as the sacrospinous and sacrotuberous ligaments. The articulation of the lumbar vertebrae allows for flexion, extension, and lateral flexion, but the orientation of the articular processes largely prohibits rotation at the lumbar region.