What is it and how to achieve it

"The posture for Yoga should be steady and comfortable"
(Patanjali's Yoga Sutra, 2.46)

Gnōthi Seauton
"Know Thyself"
(The Delphic maxim carved into Apollo's Temple at Delphi)

There is a common belief in yoga that there is just one correct alignment for every pose, one right way that everyone should learn and follow. But this is a misconception. While it is true that there are certain key safety rules that should be followed in order to avoid trauma, the "correct" way to perform an asana may be different for different people. The right alignment for one person might not be good for another who has a different body constitution, weight, physical proportions, or trauma history.

Our problem as humans is that we believe in - and seek to find - a one and only correct way of doing things. But, inevitably, this leads to problems and traumas that are physical, emotional, and spiritual. Rather than search for the one and only correct alignment, each of us needs to get to know ourselves, and only by knowing ourselves can we find our individual way of practicing and of Being here.

This is why a yoga teacher cannot know better than you what the correct posture for you, personally, should be. Only you can feel your body from the inside and analyze how a pose feels, how long you can and should hold it, and when should you leave it.

Instead of being fanatic about "proper" strict alignment of the visible exterior, such as what one learns at an Iyengar class, it could be better to focus on finding the energetically correct way of doing a yoga pose for you.

An energetically correct pose is a position of your body where you have found your inner and outer balance, feel physically strong yet relaxed, energetically even and full. Where you feel good, without pain or discomfort. Where you could actually meditate in the pose, it feels so harmonious to be there.

If, in a pose, you feel overexcitement, inner tension, or even euphoria, it is not yoga or yogic state of mind, and this practice will not deliver you the results yoga is capable of delivering. If your heart is pumping like crazy or your knees hurt, there is not much benefit in this yoga practice. If you want to have a cardio workout with a pumping heart, you may want to do something besides yoga.

It is important to find your own energetically-correct position in the asana - then it is Yoga. Though on the outside, this can look different for different people. And it is quite possible for someone to be in the alignment that is most energetically correct for them while, from the outside, looking "not correct."

I don't say that you should neglect the common sense and all the rules of yoga practice. It is very important to know safety basics and understand yoga anatomy. And actually because of the knowledge of yoga anatomy and understanding of how different our skeletal structures could be - watch Paul Grilley's Yoga Anatomy course if you haven't yet - we know, that it is also quite possible that the generally-accepted "correct" way of doing the pose may actually harm this person because of his or her unique skeletal structure or muscle condition, so one should not force himself into so-called "correct" position if it is not the most suitable position for them personally.

In the end, the main definition of yoga practice will always be what Delphic maxim reminds us of: Know thyself. And know yourself not from the books, but from your own experience.