One of the definitions of yoga is from the Yoga Sutras:  Y..S. I.2 Yoga citta vrtti nirodha.  Which means that Yoga is the cessation of the fluctuations of the mind.  But the mind, being an intangible object, is hard to bring under control.  The journey to consciousness, therefore, begins from the outermost layer: the physical body. By stretching the body through various asanas, we can gain easier access to the mind. 

“There is a direct reciprocal connection between the body and the mind.  Every change in the body is reflected in the mind.  As the body becomes free, the mind too, attains freedom.  As the body becomes supple, the mind becomes increasingly agile.  Yogasanas teach the body to be firm and toned when necessary and soft and yielding at other times. Gradually the mind alos imbibes the dual virtues of gentleness and firmness. Control over the body eventually helps to attain control over the mind.” 

In this Chapter, Mr. Iyengar talks about one way of gaining control over the mind by understanding the similarities and differences in two different asanas; Utthita Trikonasana and Utthita Parsva Konasana. In both asanas the trunk is bent sideways and the body extends from the sides.  However, Utthita Parsva Konasana is performed with a bent front knee and in Utthita Trikonasana, the front leg is straight.  A bent leg is easier than a straight leg.  When the leg is bent, the mind becomes docile and submissive.  While a straight leg requires both the body and the mind to be firm and resolute.

In asana practice there are two types of actions: easy and difficult.  For a beginner, bending the leg is an easy, effortless action.  Keeping the leg straight is a conscious and therefore more difficult action.  The mind is naturally drawn to effortless action, while it has to be willfully drawn towards the strenuous action. 

While I agree that bending the knee in standing poses is easier than keeping the leg straight, bending the front leg to 90 degrees is also a more conscious and more difficult action. 

In the Iyengar tradition, it is recommended that you learn Utthita Parsva Konasana (bent leg pose) before Utthita Trikonasana (straight leg pose).  It is often easier to strengthen the muscles than it is to stretch them. 

This process of learning poses in yoga helps to understand how to engage the mind in undertaking more difficult tasks, thus deepening the connection of the body and mind. By keeping the mind concentrated on either completely bending or completely straightening the front leg, we learn to keep the mind on task and restrain its fluctuations.