When practicing yoga, it is important to understand the principle that is inherent to each particular posture. Knowing this principle will enable you to perform the asana or variations of it in the proper way. To recognize the principles embodied by postures seek what each asana means, what is its purpose, and what it demands of you?
For example, ustrasana, the camel pose, is a back bend performed from a kneeling position. With hands holding onto the heels, the thighs are pushed forward while the chest expands and opens on each inhalation. The principle of this pose is to facilitate the movement of breath into the chest. Space is created in the chest by stretching the intercostal muscles in the pose, and the whole front of the body is opened up. In ustrasana you can experience the feeling of the breath down the entire frontal line of the body.
In the words of T. K. V. Desikachar, The breath is one of the best means for observing yourself in your yoga practice. Bring awareness to how your body responds to the breath, and how your breath responds to the movement of your body. The breath should be your teacher. Your natural elasticity in the body enhances as you breathe. As your body moves your breath moves, thus the breath and body become one movement, one process. The key to right practice and the appropriate variations of an asana is to maintain the link between breath and body. By means of the breath you can be with the whole body and observe the unfolding of an asana. Benefits of the asana deepen when the breath is smooth and has continuity. So if you find yourself struggling with your body in an asana, observe your breaths.