The fourth step is the control (yama) of the life energy (prāṇa), thus prāṇāyāma (sūtra 2.49):
tasmin sati śvāsa-praśvāsayor gati-vicchedaḥ prāṇāyāmaḥ
If you are firm in it (the sitting position of the third stage), the control over prāṇa is done by interrupting the inhalation and exhalation. Through proper breathing, the yogi is able to store the cosmic life energy. Proper breathing means breathing properly and consciously throughout the day – not just during the breathing exercises. Paramapadma Dhiranandaji recommends
one focuses on breathing in and out of the larynx and imagines breathing in and out there. This allows you to breathe more calmly and to be able to absorb more oxygen and release carbon dioxide with each breath. By concentrating the breathing on the larynx (instead of on the nose), the larynx and thus automatically the diaphragm relax, giving the desired effect. In everyday life, the yogi therefore needs less strength for his activities.
With breathing exercises the yogi trains his respiratory organs and can provide his organism with new, fresh energy. Without special breathing training, humans use around five to seven percent of their actual breathing capacity. One can now imagine what energy is available to the yogi if he can expand that capacity.
The effects of the breathing exercises are very versatile. In addition to breathing capacity training, all breathing exercises have in common that they have a cleansing effect and help to control the mind. Patañjali describes this effect in sūtra 2.48 and 2.49: Success in prāṇāyāma means that the veil of ignorance is removed and the yogi receives the ability to concentrate thought.
The soul controls the breath, the breath controls the mind, the mind controls the sense organs. Through the breathing exercises the thoughts and feelings come to rest, the mind is freed of unnecessary ballast and purified. This gives the yogi the aforementioned ability to concentrate.
By the veil of ignorance is meant māyā, the illusion or delusion that veils the true nature of man and the universe. Normally, we perceive the outer world through our sense organs. The impressions of the sense organs are passed on to the brain and the stimuli are further processed through the interplay of ego and intelligence. But since the whole world is in a process of permanent change, it is not possible to recognize an absolute truth with our sense organs. In truth, every human being, every animal, every plant, and also inanimate nature is based on a divine substance. This divine substance is ātmā, the divine soul. To recognize them and thereby recognize oneself is the goal of human life. As soon as the mind becomes calm, the veil of māyā is also cleared.