As Gregor Maehle writes in Ashtanga Yoga: The Intermediate Series:
"The Sanskrit term pasha means noose. Noose refers here to the position of the arms, which are thrown like a noose around the legs. Pasha is also one of the thousand names of the Lord Shiva, who is also called Pashaye, Lord with the noose. The Hatha Yoga Pradipika starts with the assertion that is was the Lord Shiva (known in this case as Adinatha or "primeval master") who first taught yoga. What could be more befitting than to start the Intermediate Series with an homage to the moon-crested Lord who is held to be the author of yoga?"
The verbal root pash, meaning to bind, also generates the word pashu or beast. Yet pashu might also refer to humans enmeshed in samsara, bound to suffering within what seems to be conditioned or impermanent existence. Pashus remain unaware of the underlying unchanging nature of existence and their own place within it.
As the physical posture that begins the Second Series, pashasana maps onto utkatasana--the first posture of the primary series. Both postures require an uncomfortable amount of strength and stamina, most especially of the leg and buttock muscles. Like parivrta parshvakonasana, revolved side angle of the standing series postures, pashasana is a flexion twist, in which the spine stays in flexion as it twists rather than extension, which might prove more than unusually impossible. Requiring as much strength as flexibility, pashasana commences the Intermediate Series with a reformulation and intensification of some of the twists of the Primary Series, alerting the practitioner to the strength she will need to support herself in the backbending postures to come.
In pashasana, we remain quite literally bound to, or, perhaps more precisely, bound in suffering. The posture is as such seemingly a performance of our existence as beings who suffer. Yet it also affords us the opportunity to reexamine our relationship with samsara--the cyclical pattern of birth, life and death, a cycle of transience that may refer both to one life and the many components of one life (desires, identities, beliefs, etc.).