Supta kurmasana is one of the deepest poses of the Ashtanga yoga primary series. Although it might be very challenging to perform at first, in time this pose becomes a very meditative and comfortable position.
As the Ashtanga yoga guide, "Yoga Mala," explains, "supta kurmasana purifies the kanda. It also purifies the heart and lungs, and eliminates ailments caused by imbalances of the kapha dosha." The kanda is the hub where the spinal cord exits the spine and from which all 72,000 nadis (channels of energy which closely relate to our nervous system) originate. And according to Ayurveda, the kapha dosha is one of the three bodily humors that govern all structures and lubrication in the mind and body.
There are other methods of entering into supta kurmasana, but they require a fair amount more of flexibility in the hips. It is recommended that beginners practice with an experienced teacher who can assist with entering into this pose.