Difficult standing posture in hatha yoga
Durvasasana (Sanskrit: दुर्वासासन) or Durvasana, is an advanced standing asana in hatha yoga, with one leg raised and the foot hooked behind the neck. The similar Trivikramasana (Sanskrit: त्रिविक्रमासन) has the raised leg straight. There are seated and reclining variations including Bhairavasana. Versions of the pose are depicted in statues in Karnataka and Tamil Nadu from the 8th century onwards. Trivikramasana is described in the 18th century "Haṭhābhyāsapaddhati"; a pose close to Durvasasana is illustrated as "Trivikramasana" in the 19th century "Sritattvanidhi".
Etymology and origins.
Durvasasana is named after Durvasa (दुर्वासा), a proverbially angry sage.
Trivikramasana is named after Trivikrama, a figure in Hindu mythology whose name means "three strides". The pose is depicted in the 13th - 18th century statues of Bharatnatyam dance that decorate the Eastern Gopuram of the Nataraja Temple, Chidambaram in Tamil Nadu. 
The 18th century "Haṭhābhyāsapaddhati" verse 81 describes a pose that it names Trivikramasana with the words "Place a foot on the neck and stand up".
The 19th century "Sritattvanidhi" describes and illustrates a pose that it names Trivikramasana, but which the yoga scholar Norman Sjoman states is Durvasasana.
The name Bhairavasana comes from the Sanskrit "Bhairava" (भैरव) meaning "terrible, formidable". "Kāla" (काल) "Bhairava" is the Hindu god Shiva in his aspect as the universe's destroyer. The pose is illustrated in the "Sritattvanidhi" under the name "Aṇkuśāsana" (अण्कुशआसन), from Sanskrit अण्कुश aṇkuś, an elephant goad.
Description.
Durvasasana is an advanced standing balancing pose with one leg behind the neck; the hands are held together over the chest in prayer position. As well as rating the pose of difficulty level 21 (out of 60), B. K. S. Iyengar states that it is difficult to balance in the pose, and recommends using a support to begin with. In Ashtanga Vinyasa Yoga, the breathing in the pose is stated to be either natural or ujjayi.
Variations.
Eka Pada Sirsasana (literally "One Foot Head Pose", also called Foot behind the Head Pose) is the seated variant of Durvasasana. One leg is stretched straight forwards along the ground; the other is hooked behind the neck. The hands may be placed in prayer position. It should not be confused with Eka Pada Sirsasana, a variant of Sirsasana (headstand).
Trivikramasana has one leg stretched straight up beside the body; the ankle is grasped with one hand. The other arm is stretched straight out sideways.
Supta Trivikramasana or "Supine Splits" is the reclining form of Trivikramasana. In "Light on Yoga", Iyengar marks it as a difficult pose at level 39.
(Supta) Bhairavasana, also called Aṇkuśāsana or Elephant Goad Pose, is the reclining form of Durvasasana. The hands are held in prayer position in front of the chest.
Kala Bhairavasana (काला भैरवासन) has the body balanced one side up, on the straight leg and the arm on that side, somewhat as in Utthita Vasisthasana, but with the other leg behind the neck. 
Chakorasana, from Sanskrit चकोर "Chakora", a partridge, is an arm-balancing variant. Both hands are on the ground, the arms straight; one leg is hooked behind the neck; the other leg is stretched up; the body is suspended from the shoulders.
References.
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