Yoga teachers emphasize correct breathing during asanas or postures and that is for a reason. Correct breathing helps you enjoy a posture, hold it for a comfortable duration and increases the benefits of the posture.
Let’s start with breathing in general. Typically a human being breathes in and out in about 3 seconds – way too short for actually realizing the real benefits apart from life sustenance. The lungs hardly moves during such a typical breath. In olden times, Yogis or sages would measure the duration of a person’s life by the number of breaths. Naturally, it goes to say that the longer the duration of one’s breath, the longer the person will live. Hence the emphasis on longer breaths in all texts. Pranayama or breath control is actually changing the way you breathe so that you can maximize the real benefits. The benefits are more absorption of oxygen, more collection of wasteful gases and their expulsion. When one can expel all the unnecessary breath-matter such as carbon-dioxide, there is more space for one to take in more oxygen.
Combining correct breathing during asanas help one relax parts of the body much more easily. When you take a deep breath, the muscles inside stretch and in the process relax. This has a cumulative effect – the relaxed muscles can absorb more and more of the nutrients that the blood stream brings them. When you are performing a posture such as Hero, you invariably tighten certain muscles in the body such as those in the legs. The fact that you practice sustained deep breathing and become aware of the leg muscles help direct the vital energy that the breath brings in and does the magic of relaxing those muscles.
The rules of breathing during postures are quite simple. When you stretch you inhale, when you compress/collapse you exhale. For e.g., when you lift up your arms in Standing Mountain or during a back-bend in Sun Salutation, it is natural that you inhale. Here you are stretching your body. When you collapse your body, such as during a forward bend in the Sun Salutation or during Paschimottanasana, it is natural that you exhale. Here the body is helping you force out the air. Now comes the question, when do you actually hold the breath? The answer is rarely. There are very few asanas where you have to hold the breath and these are really advanced postures. When you perform the adavanced-locust or Shalabasana with your arms locked underneath the body, during the lift of the legs, one holds the breath. The other time when you hold the breath is not during asanas but during Pranayama and Bandhas. Holding the breath during these has the purpose of focusing and maximizing the energy delivered. However, during asanas, holding breathe actually increases the tension and causes discomfort.
So In short, during asanas inhale while you stretch and exhale while you un-stretch.