It appears that Yoga folks often talk about the effects of Yoga Āsana on the spine in Yoga yet the reality is more based on the effects of Yoga Āsana on the external aspects of the structural form. It has also been an observation over some four decades of teaching that the two can get confused in terms of assessing developmental progress within the practice of Yoga Āsana.
Furthermore it appears that it is possible to work the body into very ‘advanced’ Yoga Āsana yet observe that the spine is not deeply influenced, for example with the hips and shoulders or lax joint ligaments facilitating the impression of the form. Hence the viniyoga of Yoga from this perspective is to start with the spine as the primary priority with the limbs the secondary priority. Thus the principles of modification of Yoga Āsana are from the perspective of allowing adjustments to the limbs in order to facilitate a deeper more profound impact on the spine.
For example it is possible to perform Yoga Āsana such as Uttānāsana with the spine relatively inert or even fixed into a lumbar lordosis with the impression of movement being conveyed through the range of movement manifesting via the hips, buttocks and hamstrings.
This means that the starting point for any observation has to be what is happening in the the spine. These are part of the criteria that my teacher taught me and are essentials to be satisfied, then the work can shift to include other areas as part of the focus. However the reference for whatever you choose to include as a focus that moves the attention away from the spine is evaluated by the affect, one way or another, on the spine.
Woven into this precious teaching and understanding are also the energetic processes inherent in Haṭha Yoga and along with the spine so often sadly neglect
Hence my teacher taught me that it is more vital to observe how Yoga Āsana influence the Spine rather than the more generalised impression of how Yoga Āsana influence the Body. In other words assess the effect of the Yoga Āsana from the inside out rather than the more usual impression arising from observing from the outside in.