In yoga we tend to tend to focus our movements within just three planes of motion. This simple exercise takes the many small facet joints in our neck through their entire spectrum of movement.
During my recent workshops I noticed that there is still a lot of confusion about the importance of nutating the sacroiliac joints in various forms of back-bending (but also in forward bending). Here is a modified passage from my 2009 text Ashtanga Yoga The Intermediate Series that sheds light on sacrum nutation during Kapotasana. This is something that any natural backbender will do automatically but even if you do not belong to this group, the actions can be induced by understanding and learning. This will improve your backbend significantly. This passage is shortened as the original has over 5500 words.