The plough comes after the shoulderstand in our series and is a full stretch for the entire back side of the body, from the heels to the neck.
Swami Vishnudevananda (bringer of yoga to the West) on the plough:
With proper knowledge of the spine, the body can be trained to maintain its flexibility from early childhood until death. Halasana pulls the whole spine steadily, posteriorly. Every vertebra and ligament of the cervical spine region receives plenty of blood and becomes healthy. All movements of the spine directly or indirectly give exercise to various internal organs and abdominal muscles.
By practice of the yogic system, we not only retain the elasticity of youth and eliminate the abnormal growth of mineral deposits in the bones, but to a great degree we also can also regain lost youth. The elasticity of the body further depends on the state of the blood vessels and vertebral column; the gradual accumulation of sedimentary deposits, such as lime in the arterial and verous valves, owing to the impurities in the blood, gives rise to inelasticity in these parts. The faster these deposits accumulate, the quicker old age is upon us.
The plough is followed by the fish.