Pratyahara

Pratyahara (Sanskrit प्रत्याहार, Pratyāhāra) or the 'withdrawal of the senses' is the fifth element among the Eight stages of Patanjali's Ashtanga Yoga, as mentioned in his classical work, Yoga Sutras of Patanjali composed in the 2nd century BCE. It is also the first stage of the six-branch yoga (ṣaḍaṅgayoga) of the Buddhist Kālacakra tantra, where it refers to the withdrawal of the five senses from external objects to be replaced by the mentally created senses of an enlightened deity. This phase is roughly analogous to the physical isolation (kāyaviveka, Tib. lus bden) phase of Guhyasamāja tantra.
This asana prepares the spiritual aspirant for the fifth stage of yogic practice, pratyahara, sense withdrawal, which is symbolized by the tortoise. The Bhagavad Gita states, "When he (the yogic aspirant) can withdraw his senses from association with other objects, as a tortoise withdraws its limbs from external danger, then he is firmly fixed on the path towards wisdom." (2:58) Koormasana is dedicated to Koorma, the tortoise incarnation of Lord Vishnu, maintainer of the universe.