First off I am not a huge expert on Yoga, just a practitioner and someone who has experienced the benefits of following most of, or at least most of, the 8 aspects of Patanjali Yoga. Asanas being one of the 8 disciplines within yoga. Yoga is actually “Yog” a way of being, of achieving the highest in self awareness and self perfection. Simplistically put Yog is a totality of 1.social behavior , 2.inner discipline, 3.body postures that are not simply postures but a careful mind-body balance act, 4.breathing techniques, 5.control of the five senses, 6.focus or concentration, 7.meditation and finally 8.Samadhi which is the state of egolessness, of bliss. Yoga philosophy encompasses the merging of many many many things. For instance, the dualities of material versus cosmic (purush-prakriti), of the ‘gunas’=innate tendencies of humans It is too deep a subject actually. A good study would need a lifetime. Also, the vast country that India is most of our Again, I am not an expert but to my knowledge Patanjali ( From early AD/ 2000years ago) Yoga is the main source of Modern Yoga as we are (very briefly, tip of the iceberg kinds) familiar with it. It has been studied and propagated in different forms by many different scholars. Tirumalai Krishnamacharya (1888-1989) for one, yes, he is referred to as Father of Modern Yoga sometimes (though if you ask the majority of Indians will never have heard of him or Taimni) Another person who was in North India was I.K.Taimini (1898-1978) He too interpreted Patanjali’s Yoga. (Side Note: South of India has more of a tendency towards scholarly endeavours as compared to North India that too has reasons in history. North was always fighting the onslaughts from the Himalayan side whereas our Dravidian belt was cut off from the North by the Vindhyachal ranges and had more to do with pursuits of research and study.Ok: digression end) Now, why I mention the two scholars (I have read neither and am sure there are many more) their interpretation of Patanjalis Yoga Sutras will be very different based on their experiences with it. B.K.S.Iyengar (1918-2014) was related to Tirumalai Krishnamacharya and was briefly his student. Now the thing is that even Krishnamacharya changed the postures to suit individual needs, as I said there are many many factors, based on a body’s doshas, gunas, prakriti etc…an individual manifestation of any given asana will differ. Also, please know that there are layers and layers of work that is needed to get to the so called difficult asanas. Basic ones can be done by almost all but the ones that seem very tough need just that, a tough mind and a tough mental determination. So why are they so tough and like the ‘mallakhambh’ which is a sport actually. You see, many a times men would congregate at the mallakhambh (a place to practice the pole excercises) and there it would be convenient to teach the Yoga asanas also, both the mallakhambh and yog needing strength of both body and mind. But, honestly I doubt if the Iyenger yoga was developed for the boys at the mallakhambhs. The toughness of the asanas would surely be more to do with mind games than anything else. Pushing oneself to run and think being comparable…….Brings us to the question…Is it for tough people not satisfied with the simple asanas…not so simplistic as all that I would think….It is not so much for tough people more the tough minds as the complicated asanas, teach the doer the finer tunings of the body, brings one closer to creation and an understanding of meaning of being, simplistically put help give the practitioner a deeper lesson in patience, an insight into what the Gita syas..…karmanye vaadhikaaraste…ma phaleshu kadachanah….keep at the work in the right manner…following the correct principles (breath is essential part of the asanas)….. do not work for the results, for the tougher asanas…just concentrating on the moment with the simultaneous awareness of the journey, the path to reach the final pose (the goal) and stay in it and in doing so savour the path too. It’s an understanding of life and beyond, that's 'yog' or yoga as it is popularly called,…..and as I said we are still at the tip of the iceberg.