Kaya Siddhi is a kind of yogic practice pertaining to the body. While describing the effects of the practice of niyamas (observances) such as purity, contentment, penance, study, and devotion to God, Patanjali declares in his Yogasutra that due to the practice of Tapas (austerities) the adept achieves powers pertaining to kayendriya siddhi (the body and sense organs) (II.43.).
This siddhis are of eight different kinds and hence are also called Ashta Siddhi. They accrue as a result of the purification of the body by removal of impurities through tapas. Patanjali calls them kayasampat (III.45). Kayasampat is achieved when the five basic elements of prithvi (earth), jala (water), tejas (fire), vayu (air) and akasha (space) are conquered by intense concentration, meditation and absorption (samyama) on them. By practicing samyama on the relation between the body and the ether or by acquiring through meditation the lightness of cotton fiber, the yogi can fly through the air (II-43).
The eight siddhis or occult powers pertaining to the body are as follows – anima (the power to make oneself very small), laghima (the power to make oneself light), mahima (to make the body large), garima (to increase the weight at will), prapti ( to achieve the fulfillment of any desire),vasitva (to control all beings, animals and so on), and isitva (to control the emergence and dissolution of anything).
None of these powers is to be misused. In fact, they have to be given up. “By giving up these powers, the seed of evil is destroyed and liberation follows”. (II-51).