I want to revisit a (quasi rhetorical) question I posed a couple weeks ago which considered how much of yoga is really about the asana. A good answer to that is complicated and probably more than I can discuss adequately here, but the short version is "it depends." A more concise question may be to say how much of YOUR yoga is about the asana. Although, the answer to that also depends. It depends on various factors, but very much on your ultimate intentions as a practitioner (i.e. are you seeking a means to physical health, mental health, spirituality, relaxation, stress-relief, socializing, scholarly studies, etc?).
Let's, for the sake of this discussion, assume your practice closely follows the tenets of Patanjali's Classical Yoga as expounded in his Yoga Sutra. After all, we are attempting to understand the wisdom of Mr. Iyengar's Light on Yoga, and Mr. Iyengar certainly considered himself to be a student of Patanjali. And if we want to better understand our own teachers, it is to our benefit to try to understand theirs. In that case, the answer to the question (how much of yoga is really about the asana?) is "not much." In order to really understand why that is, we have to become familiar with the premises of Classical Yoga.
Classical Yoga is composed of eight stages of practice (asana being one of them) which culminate in the mind's liberation from its incessant turmoil. That is a tall order, and a murky one to boot. Basically, Patanjali sees the "human problem" -- the seemingly innate suffering that unifies all of human existence -- as being one of uncontrolled mental agitations, and, if said mental agitations could be reigned in, then the result would be a transcendent joy and wisdom. But, because the mind if complicated, illusive, and purely conceptual, affecting it is extremely difficult. Therefore, Patanjali says, rather than diving immediately into the (dys)functioning of the mind, start with something a little more accessible -- your body, for instance.
In regards to the stages of practice, mastery of the poses is neither the ultimate objective, nor the groundwork. In fact, technically speaking, "mastery of the poses" in the sense of its presentation in something like Light on Yoga (or nearly every modern public yoga class) doesn't have anything at all to do with Patanjali's idea of asana. The word asanam literally means "seat" or "customary ways of sitting." Patanjali could not care less if you have mastered the Triangle pose or the Shoulderstand or the Half Bound Cross Footed Balancing Cockatoo; his only concern is that you are capable of sitting in a stable and comfortable position for extended periods of time. So then why all the poses?
We're getting there. But first, if asana isn't the most preliminary stage, then what is? Asana is actually the third stage in Patanjali's system. The first is yama and the second is niyama. If a practitioner wishes to obtain the mental harmony that yoga provides, then she must begin by redirecting her worldly inclinations; that is, shifting her perspective away from those tendencies which tether her to the phenomenal world. The word yama means "restraint," so Patanjali's first and second stages are restraints or limitations related to human behavior. In other words, they're ethics because, in order to be a successful yogi, one must be well-behaved, both internally and externally. There are five yama and also five niyama. The yama focus on optimal ways of interacting with others and our external environments, while the niyama are more personal in nature and reflect a healthy relationship with our more internal states. Tomes of commentary have been written about each of them, and this is not the place for additional elaboration (if you want help finding good resources for this material, just ask).
In brief, the five yama are: (1) never injure or hinder another living being (ahimsa); (2) always be truthful in thought, word, and action (satya); (3) don't take what doesn't belong to you (asteya); (4) uphold moderation in all actions, engagements, and consumptions (brahmacharya); and (5) only use/amass those things which are absolutely necessary to your well-being (aparigraha).
And the five niyama are: (1) maintain good physical health, nutrition, and cleanliness (saucha); (2) cultivate contentment, graciousness, and gratitude toward all that you have and are (santosha); (3) cultivate passion, courage, and dedication toward that which is most important to you (tapas); (4) study with the intention of garnering universal truth and wisdom (svadhyaya); and (5) be devout toward that part of yourself that you know is "more-than-just-a-body" (Ishvara pranidhana).
The role of the yama and niyama is to help establish one's character which is important because it shows a willingness and ability to overcome strong (likely destructive) temptations. Everyday life -- its challenges, misfortunes, and inequalities -- can easily lead one into patterns of moral gray-zones, and calming the mind's turbulence is dependent upon a steadfast intention to honor and respect all living beings (yourself included). However, yoga is not about conscience-building; it's about Liberation and Authenticity in the strongest sense. So, while demonstrating that you are capable of playing well with others while taking good care of yourself is necessary, it's only the very beginning.
From here on out, the practice becomes increasingly more internally focused with each subsequent stage. Whereas practicing the yama and niyama lead to freedom from worldly distractions -- envy, greed, discontent, dishonor, etc. -- practicing asana leads to freedom from your own physical distractions -- fatigue, disease, fragility, immobilization, and injury. Over hundreds or even thousands of years, the asana have evolved into elaborate (and sometimes conflicting) methods of positioning the body which undoubtedly develop strength, flexibility, stamina, vitality, resilience, and grace. All of which are good things. But remember what the word asanam means: "seat," "customary ways of sitting." The point is to condition the body to sit. Literally. If the back is weak, the hips are stiff, the knees ache, the shoulders slouch, or the head hangs, sitting still for more than a few brief (and probably unsettled) moments is nearly impossible. So the yogi uses her asana practice as a means of achieving physical firmness (sthira) and steadiness (sukha). We'll see why soon.
The poses target the outer-most body first and most obviously; namely, muscles, tissues, and bones. But in the meantime, they are also having an equally profound effect on the more subtle layers of our physiology; that is, our cells, nerves, and glands. The fact that yoga targets our bodies wholly -- outside and in -- leads directly into the fourth stage of practice: pranayama.
Prana means breath, life-force, energy. Ayama still means restraint and also extension. So, pranayama is the practice of literally controlling one's breath, and, more figuratively, one's spirit. The various pranayama exercises comprise an entire system of practice in addition to asana (Iyengar's manual covering this material is called Light on Pranayama). Most practitioners will never venture beyond the very basics -- Ujjayi, possibly some short retentions, and maybe Kapalabhati. Anything more advanced than that tends to exceed the average student's interests and commitments as it is a quite challenging stage of practice which really demands dedicated focus as well as very close supervision from an experienced instructor. It also requires that you be able to sit for extended periods of time, hence the previous stage (asanam = "ways of sitting"). In fact, stages four through seven (pranayama, pratyahara, dharana, and dhyana) all require a body which is willing and able to endure prolonged periods of quiet and still sitting, so it's really no wonder that one entire stage of practice, not to mention the one most well-known and widely practiced, is nearly exclusively focused on training the body to do just that. Without the physical capacities which come from asana, one would almost certainly not be able to withstand the difficulties of the proceeding stages.
Asana and pranayama work together to tame the body, physically and physiologically, in order to suppress what is otherwise a perpetual slew of distractions and demands. If your most-physical self is in any way unhealthy, imbalanced, or disturbed, your mind cannot rest, and yoga's promise of ultimate liberation will forever elude you. However, please don't hear that and think to yourself Well, I'll never practice advanced poses or breath-control, so maybe I shouldn't do this at all or Oh no, now I've got to find a real Guru to teach me how to sit, breathe, and think or I'll never experience personal contentment. That's not the case. Even if advanced asana or pranayama seem beyond your yogic scope, your practice is absolutely still important and valuable. Yoga is beneficial in its most elementary forms, and you do not have to be a contortionist or a shaman or a vegan or a Hindu or a philosopher or an astrologer to be a good Yogi. You only have to be willing to conform your practice to fit your needs, and not the other way around. All eight stages have elements which are more suited for beginners as well as those which are better saved until one has gained more experience. Even a modest practice can include some aspect of each stage, or even just a few of the stages, so that your practice provides you with exactly what you most need. And that is infinitely better than no practice at all.
Once the body has been brought under control through asana and pranayama, the mind is actually more distracted than ever! Because it no longer has physical forms and functions to worry about, the sensory world kicks into overdrive. The fifth stage of practice is called pratyahara, and it teaches the yogi how to control her senses; namely, sight, taste, touch, hearing, and smell. The purpose is to continue the disconnection that is developing between the Self and the surrounding world. Our senses -- as valuable and inherent as they are to the human experience -- are the embodiment of external awareness and mental disturbance. In order to entirely free the mind, sensory perceptions must be challenged. Pratyahara is the practice of dissociating from the senses. It is not about turning them off or lacking them completely; instead, it is about learning how to control them rather than allowing them to be controlling.
At this point, the mind becomes the primary focal point more than ever before, and stages six and seven are practices of mental concentration (dharana) and meditation (dhyana). The words "concentration" and "meditation" are often used interchangeably, but for the yogi they are two distinctly different levels of experience and achievement. Explaining just what each one is and how they differ is worth saving for another discussion. It's enough at this point to understand that one's consciousness is slowly brought more and more inward until neither the external world, nor the very body we inhabit, are distracted or agitated; only pure, unadulterated thought is left. And not for long, because that too will be mastered. The eighth stage (Samadhi) is the experience of an entirely liberated state of existence wherein your body thrives, your heart is content, your mind is quiet, and your consciousness is dissolved. YOU in the grandest sense -- the part-of-you-that-is-more-than-your-body -- are incomparably alive. And that is what yoga is really about.
When you put this all together, you have what Patanjali calls "Ashtanga Yoga" because it consists of eight (ashta) limbs or stages (anga). The type of yoga practice founded by Patabbhi Jois which features vigorous vinyasa flow and is called "Ashtanga Yoga" shares its name with the Classical Yoga system because Jois, like Iyengar, was a student of the Yoga Sutra. Despite the many differences between Iyengar and Ashtanga yoga practice, their most basic premises are the same; namely, that yoga is a science and a philosophy capable of promoting incredible physical and psychological transformations if one is willing to be sincerely disciplined and humble.
So again how much of yoga is really about the asana? It still depends. For Patanjali and his eight-stage system (Ashtanga Yoga), it is necessary only as a means to a much, much bigger end. The asana play a very prominent role because our bodies are very tangible and manipulable. We can touch them and change them concretely, which makes them (relatively) easy to understand. We can, through our physical exercises, get a glimpse of all the other aspects of practice: the respect and compassion that comes from following the yama and niyama, the vitality that comes from pranayama, the autonomy of pratyahara, and the clarity of dharana and dhyana. We can even, in brief moments of genuine respite, during an especially pleasant Savasana, for instance, feel what Samadhi must be like. In short, we practice the poses in order to enliven our human experience and to illuminate those parts of ourselves which are superhuman.