On page 46 of Yoga: The Path to Holistic Health by B.K.S. Iyengar there is the quote:
"An asana is not a posture which you assume mechanically. It involves thought, at the end of which a balance is achieved between movement and resistance."
I read this to some of the classes during the week that we compared the two Utthita trikonasana (triangle pose) photos -- one from the Yoga Journal calendar for the month of May, and the other either from The Path to Holistic Health, or from Yoga: The Iyengar Way.
Yoga Journal 2010 Calendar
Compare the two Trikonasana photos on this page. They're both nice poses -- arms, legs, and trunk are extended, and both have hands on the floor. But there are also some differences.
In the first photo, the Yoga Journal Trikonasana is a decent pose. But notice that the stance is narrower than in the Trikonasana on the Yoga: The Iyengar Way book cover.
The second picture is of a gorgeous pose! The wider stance makes it more possible to bring the whole trunk into better alignment.
Look closely at the first picture. You'll see that her left leg (the back leg) thigh is rolling down slightly. This rolls the left hip downward, narrowing the front of the pelvis. This results in the chest rolling slightly toward the floor. You can see this in the v-neck of her top. The midline of the front of her trunk is turning slightly downward, but her gaze is upward. There's a disconnect between the spine and the neck.
In the second picture, her stance is wider, and her left hip rolls back rather than forward, opening up the front of the pelvis. This makes it easier to turn the whole trunk more fully in the pose. The turning of the face results from an even turning of the spine and neck. There's no disconnect between the two.
In the first photo, the turning in of the left leg may be partly from a stiffness in the hips, or of the placement of the left foot. We do often teach to turn the back foot in a little because it does make the pose easier to enter for beginners. But if we turn the foot forward, as in the second photo, it makes it easier to keep the left thigh and pelvis facing forward. That makes it easier to turn the whole trunk correctly.
Also if the woman in the first photo took her right hand to her shin or to a block, she'd be able to turn her trunk better.
Yoga: The Iyengar Way
If we do this pose mechanically, we never figure out where we are going wrong. We turn the right leg out, the left foot in, straighten the legs, extend the trunk and take the arm down. That is fine for beginners (and we can be beginners for a long time!). But to grow in our poses, we need to start looking at them in a more thoughtful manner. We need to look at what needs to move and what needs to stay put. The right leg turns out, and if we're not paying attention, the left leg goes with it and turns inward, especially if we have tight hips. We need to learn to resist the left leg from turning in.
If the hips are tight or the hamstrings or ankles are tight, we may not be able to have a wide stance in the pose, but then it's best to not take the hand to the floor. That just collapses the chest downward. It takes thoughtfulness and inner reflection to see this in ourselves. We need to develop this thoughtful reflection to improve our poses.