In Western anatomy the spine provides support, structure and movement for the upper body, it carries the head and houses and protects the spinal cord. In Eastern anatomy it is the main channel known as Sushmana, or Sushumna, which carries the primal life force energy, the Kundalini, upwards, cleansing, purifying and connecting us with the Infinite.
In Kundalini Yoga, one of the first things you notice is how much of the kriyas are done in a sitting posture. We leave much of the downward dogs and tree poses to Hatha traditions. In Kundalini we are focusing primarily on raising the energy, and of course the spine is therefore often our main focus. In the modern world many of us spend far too much time sitting - in cars, at desks, on sofas - and this causes huge postural, and therefore energetic problems.
Spinal flexes are a great way to loosen up all that tension from long hours of sitting, increase blood flow to the muscles and nerves, encourage the circulation of the cerebro-spinal fluid which nourishes and feeds the central nervous system (spinal cord and brain), and open up the subtle channel of Sushmana for our life force energy to flow.
I can thoroughly recommend spinal flexing for a quick energy burst. It has helped so many of my students with lower back pains to ease the tension and reverse wear and tear injuries that were beginning to seriously erode their quality of life.
The yogis relate flexibility of the spine directly with longevity - the more flexible your spine, the younger you feel. And it's true! So take care of your spine with these simple exercises, so that it can take care of you for as long as your soul wishes to stay here on earth, in a body that is lithe, vital and strong. Wahe guru!