Samkhya-Yoga-Kundalini
Tasmin-sati śvāsa-praśvāsayor-gati-vicchedaḥ prāṇāyāmaḥ
When this is achieved [the yogi should practice] prāṇāyāma, [which is] interrupting the motion of the breath coming in and going out.
When this has been achieved refers to becoming steady and relaxed in asana. Interrupting the motion of the breath refers to the practice known as kumbhaka, or stopping the movement of breath, either at the end of inhalation or the end of exhalation. Patanjali defines pranayama in terms of kumbhaka, which gives the sense that he regarded kumbhaka as important.
Caution! Well respected teachers have said that this should not be attempted until basic inhalation and exhalation practices have been mastered.
bāhya-abhyantara-stambha-vṛttir-deśa-kāla-saṅkhyābhiḥ paridṛṣṭo dīrgha-sūkṣmaḥ
Its [the breath’s] modes of operation are outward, inward, and fixed, when observed by place, time, and number, [it becomes] long and subtle.
There are differing explanations of this sutra. The simple and seemingly obvious one is that the three modes of operation refer to inhalation, exhalation, and kumbhaka. The older, more traditional translators are adamant that this is not the case, that inward and outward refers to the places where kumbhaka is performed, and that stambha is not kumbhaka but a practice of suppressing the breath in which breathing is sometimes suspended for long periods of time.
Likewise, there are differing explanations as to the meaning of place. There are other Hindu texts that refer to stopping the breath in different places. Based on that, breath can be understood as a metaphor for prana or energy, which can be consciously directed to various parts of the body. Breath can also be stopped physically in different places, such as chest, ribs, and diaphragm.
Time is generally understood to mean the relative amount of time spent in inhalation and exhalation, and is usually thought of as lengthening or shortening the breath. Number is understood to be the number of repetitions of a particular exercise. To be observed refers to the ways in which breath can be regulated. The traditional translators take “long and subtle” to be the desired results when breath is regulated as prescribed. Others see it as referring to time and number, in a relation of more or less.
bāhya-abhyantara-viṣaya-akṣepī caturthaḥ
The object transcending the inward and outward [movement of the breath] is the fourth.
What is the object that transcends the inward and outward movement of the breath? It is prana, the subtle energy for which breath is a metaphor . This subtle meaning of breath is the fourth mode of operation, in addition to the three mentioned previously. The nature of prana will be discussed elsewhere.
Tataḥ kṣīyate prakāśa-avaraṇam
From that, the covering of [the inner] light falls away.
By the inner light is meant the spiritual light that represents the param-atma, the supreme soul or the soul of the universe, and which is present in all beings as jiva-atma, the individual soul. the author is stating that the yogi develops the ability to see this light through pranayama. This should be understood as a form of meditation practice that requires no other method except for pranayama.
Dhāraṇāsu ca yogyatā manasaḥ
And [the yogi acquires] fitness of the mind for dharana.
The author is stating another benefit of pranayama, which is fitness of the mind for concentration. By this is meant the calming of the mind, slowing down the constant churning of thoughts, of sense impressions, and the desire for the enjoyment of the senses. The yogi must calm the mind in order to gain the ability to focus. This is accomplished through pranayama.
In the pranayama described here, the yogi must be seated comfortably and relaxed. The limbs and the senses are brought to a state of being still through asana. Then the yogi can begin to focus on the breath, and when he is ready, to interrupt its movement. At first, kumbhaka may be momentary, simply noting the point at which one movement of the breath ends and the next begins. With practice it can be lengthened as desired.
In modern yoga practices, pranayama is sometimes done along with asanas. The different asanas can be used to direct prana to various parts of the body. This is said to be accomplished through nadis, which are passageways through which prana can flow. This is analogous to the nervous system, the circulatory system, the digestive system, etc. In this type of pranayama, the yogi in asana must be relaxed enough so that he becomes aware of and takes control of his breathing. The movement of the breath is coordinated with movements of the body, and when the asana becomes still, focus shifts to the breath. In this type of pranayama, the movement of the breath is not interrupted or stopped.