Form is best thought of as top-down: a holistic knowledge of a pose -- visualized, then flowing down to body areas, such as legs, and to individual parts like ankles and toes. The intent is to know what is important for the pose, in priority order for attaining the goals of the asana. For the individual: there is a strong need to truly internalize and understand that everybody performs every asana differently, either a little or a lot, for various reasons: personal goals, body type, age & physical condition, other individual characteristics.
The concept of standard 'correct' form for asanas is a 20th century invention. By contrast, in yoga foundational texts (see Hatha Yoga Pradipika page) asana descriptions are succinct (one or two sentences), frequently vague, and inconsistent (as Sanskrit translations vary). Asanas are seldom illustrated. They are, however, described adequately for determination of body parts utilized, body systems affected, and prana (life force, breath) flow to be established. The specific form for a particular asana is open to interpretation, as is necessary for an asana to be personalized appropriately for an individual. Individuals learn and master asanas through experience and teachings from yogis, developing and growing their yoga practice to fit their bodies and their lives.
Are there reference sources for how to perform asanas? Sure -- countless books and videos, each with its own approach. They fall into a broad range. At one end are rigorous approaches, allowing very little deviation -- there is a uniform goal for basically all people -- precise position & alignment are described for every joint, limb, etc. The other end of the spectrum is a very general approach -- stick figures as the only written guidance -- specifics are learned from a guru or teacher, as appropriate for the student. Which, of course, is the ultimate objective: students’ form is for their bodies, their goals. In A.G. Mohan's words, "A teacher should adapt each posture and each asana to suit the needs of the student."
There are essentials of a pose -- by individual -- and there are forms to achieve benefits without risking injury. Variations in form are due to individuals and to goals -- things to work on plus variety that develops capabilities in different ways -- different rotations, different muscles, and so forth. Be aware of general rules, but know your body. This means knowing asanas, human anatomy & physiology, then combining with specifics of an individual. General know-how is morphed into individual know-how.
Individuals need to feel the asanas, feel body parts working. Asanas should be physically comfortable as well as stable. This is called Sukha -- the other bedrock principle of asanas. Sukha refers also to mental comfort and peace. In total, while performing asanas, you should be focused, disciplined, comfortable and calm while firm as a sequoia and steady as a rock: Sthira and Sukha.
Look to the masters for practical guidance. Here are images of six basic asanas by four masters: Krishnamacharya, Iyengar, Jois, and Sivananda. Their form is the right form for them. We should strive to follow their example of self-knowledge.
