Śivasaṃhitā 3.92

Ugrasana (severe, intense posture — ugra meaning terrible, powerful in Sanskrit) is the posture of complete posterior extension: legs stretched, hands taking the head and placing it on the knees. In the contemporary yoga tradition it is known as paścimottānāsana (western extension, the back). The text uses both names as equivalents, indicating the posture was known under different names depending on the school.

Vāyuprakopana (to excite, stimulate air movement) describes ugrasana’s main effect: the spine’s position in maximum flexion compresses the abdominal organs and intestines, stimulating the apāna vāyu that normally descends downward. By reversing this pressure, apāna is forced to ascend — a movement the text calls apāna vāyu… guda mārgena (apāna through the anus path).

Six months of daily practice is the time the text prescribes for mastering ugrasana and its effects. The «destruction of lethargy and uneasiness» (tandra and māndya) are the two most common obstacles of yogic practice: the first is drowsiness during meditation; the second is laziness before practice. Ugrasana, with its vigorous action on the autonomic nervous system, eliminates them directly.