This is an excerpt from Yoga Anatomy-2nd Edition by Leslie Kaminoff & Amy Matthews.
Axial Extension, Bandhas, and Mahamudra
The fifth spinal movement, axial extension, is defined as a simultaneous reduction of both the primary and secondary curves of the spine (see figure 2.36). In other words, the cervical, thoracic, and lumbar curves are all reduced, and the result is that the overall length of the spine is increased.
Because the primary and secondary curves have a reciprocal relationship, which is expressed in the natural movements of flexion and extension, axial extension is “unnatural” in the sense that it bypasses this reciprocal relationship by reducing all three curves at once. In other words, axial extension generally doesn't happen all on its own; it usually requires conscious effort and training to accomplish.
The action that produces axial extension involves a shift in the tone and orientation of the breathing structures known as the bandhas. The three diaphragms (pelvic, respiratory, and vocal) and their surrounding musculature become more sthira (stable). 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.
A seated posture named mahamudra (figure 2.37) adds a twisting action to axial extension. It is considered a supreme accomplishment to do this practice with all three bandhas executed correctly, because it represents a complete merging of asana and pranayama practice.