Axial extension is a spinal movement defined as a simultaneous reduction of both the primary and secondary curves of the spine. In other words, the cervical, thoracic and lumbar curves are all reduced, and the result is that the overall lenght of the spine is increased.
Axial extension is "unnatural" by reducing all three curves at once; it usually requires conscious effort and training to accomplish.
The three diafragms (pelvic, respiratory, and vocal) and their surrounding musculature become more stable (sthira), activating the 3 reciprocal bandas.
As a result the shape-changing ability of the thoracic and abdominal cavities is more limited in axial extension.
The overall effect is a reduction of breath volume but an increase in length..
The overall yogic term that describes this state of the spine and breath is "mahamudra" or great seal, which always involves axial extension and the bandhas. It is possible to do mahamudra from many positions, including seated, standing, supine,and in arm support.
Conclusion: Axial extension involves a simoultaneous reduction of (a) the primary and secondary curves, which (b) lengthens the spinal column beyond its neutral alignment