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. Well respected teachers have said that kumbhaka should not be attempted until basic inhalation and exhalation practices have been mastered. However, Patanjali defines pranayama in terms of kumbhaka, which means that he regarded it as important. There s nothing to fear from momentarily stopping the movement of the breath, but it does take experience to know when and for how long to do it.
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. Physically, breath can be stopped in different parts of the lungs, such as diaphragm, rib cage, and the upper part of the lungs. 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.
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 sometimes understood to be the number of repetitions of a particular breathing exercise. It is common practice to measure the amount of time that asanas are held by the number of breaths taken while in the pose.
When breath is observed as prescribed it becomes “long and subtle”. This is a benchmark for the yogi to evaluate his/her practice. When the breath becomes long and subtle, the yogi can be confident that he/she is practicing correctly.
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. Fitness for concentration means calming 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 concentrate. This is accomplished through pranayama.
In the pranayama described here, the yogi must be comfortable and relaxed. The limbs and the senses are brought to a state of being still through asana. Then the yogi can begin to observe 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.