The anterior complex (AC) is a key sonographic landmark in ultrasound imaging for neuraxial blocks and lumbar puncture. If the AC is visible, the ultrasound beam is penetrating the canal through a softtissue window and a needle can follow this same path to reach the epidural or intrathecal space.
The "Spinning on the AC" maneuver utilizes this principle, to mark a paraspinous needle trajectory. This is a refinement of the scanningmarking technique for a paraspinous approach described in another video (   • Ultrasound Imaging for the Paraspinou...  ), and can enhance accuracy; especially where the AC is visible in the parasagittal oblique (PSO) view but not so well in the transverse midline (TM) view.
General advantages of using and mastering the paraspinous (paramedian) approach vs a midline needle approach are described here    • Paraspinous (Paramedian) Approach to ...