[3] Viparita Karani, "the Inverter", holds the head down and the feet up for hours at a time, so as to cause gravity to retain the prana.[5] Hemacandra's 11th century Yogaśāstra names it Duryodhanāsana ("Duryodhana's pose") or Kapālīkarana ("head technique"),[6] while the 18th century Joga Pradīpikā calls it Kapālī āsana, head posture; it is number 17 of the set of 84 asanas described and illustrated there.[9] In his Light on Yoga, B. K. S. Iyengar uses a forearm support, with the fingers interlocked around the head, for the basic posture Shirshasana I and its variations; he demonstrates a Western-style tripod headstand, the palms of the hands on the ground with raised elbows, for Shirshasana II and III; and other supports for further variants.Iyengar names and illustrates ten variants in all, as well as several preparatory and transitional poses.[11][12] A variety of other asanas can be used to build the required upper body strength and balance.