When addressing the back, the Active probe tip should be touching or closer to the nerve root emerging out from between the vertebrae of the Spine; the reference tip should be placed laterally out … horizontal… or further towards the end of the extremity.  For example:
From the Sacrum / Lower Lumbar, the spinal nerve roots relay signals down into the legs. Thus, the Active electrode is placed near the Spine; the Reference electrode is placed further down the same-side leg, anywhere, all the way to the tips of the toes.
From the mid-Thoracic Vertebrae, the currents move outward from the Spine and horizontally along the rib cage on the same side. Place Active at the Spinal Nerve Root and Reference further out to that side.
From the uppermost Thoracic vertebrae, the currents move down the arms, all the way to the fingertips. The probes/electrodes are placed accordingly:  Active at the Spine; Reference toward the fingers.
From the neck spinal nerve roots, Active at Cervical Nerve Roots; Reference placed on same side out into the head, face (and brain).
A proficient and knowledgeable operator enables the Acuscope to accurately ‘tap into the wiring’ by recognizing the natural flow of currents within the nervous system. It’s crucial to understand that these currents have a specific directionality. Trying to force them upstream when delivering a message is contrary to their natural flow. Instead, the operator must facilitate the movement of current in both directions, following its natural course.
Once you understand this, you can better assist the Acuscope in doing best “what it knows how to do.”