It’s a key part of yoga, alignment. Styles of yoga and different teachers will emphasise it differently, but you should be told how to align your body in asana regardless of style or teacher.
Take a hamstring stretch for example, like Janu Sirsasana. Fundamental to doing the pose properly is keeping the toes of your straight leg pointing up. Let the foot fall inwards, or more likely outwards, and you can put undue stress onto one particular part of the muscle group which could create a muscle imbalance (Walker, The Anatomy of Stretching, 2011). An imbalance in the hamstrings could end up affecting the knee or the hip.
I’ve been thinking about alignment in other areas recently. Especially my classes. To begin with my morning classes lasted 1.5 hours and the classes later in the day lasted only 1 hour. There were similarities, all the classes might focus on the same group of poses or same actions, but the styles of class were quite different.
I’ve decided to align the classes so that each class now is 1 hour long and follows the same style. The shorter classes are more active and I spend less time demonstrating poses and instead get people into poses then move around correcting as necessary. An hour and a half can seem like a long time if you’ve never done yoga before and people lead busy lives so hopefully keeping classes to 1 hour means more people can enjoy yoga.