Abhinavagupta in his Śrī Tantrālokaḥ (chapter III) says (not verbatim), “How does the beautiful beloved (Parāśakti) be separated from Her Consort, as riding on the Consciousness, if the Supreme Consciousness (Śiva) does not join Her. That, which is expected with the splendour of Bhairava (Śiva) is Parāśakti (the Supreme Power of Śiva) expands as kula (expansion of Śakti into worldly process), where She is known as Kaulikī and with whom Paramaśiva always rests. During this Divine Union, vibrations take place and this is how the world appears. This is also known as the mercy of Śiva, also known as Aghora (aghora also means non-terrific. There are different interpretations for aghora in Trika philosophy.)” Understanding this is very important, as Kuṇḍalinī is not just a force; It is Parāśakti Herself in Her subtlest form.  Therefore, arousal of Kuṇḍalinī, either through practice or due to immense and immeasurable love for Her, in both ways She alone manifests in our body and unites with Śiva at sahasrāra. Thus, Kuṇḍalinī meditation is not just another meditation, but it is about experiencing Her Absolute Grace, during Her state of Ānanda. This Bliss is caused by Her because of Her union with Śiva at sahasrāra. Since kuṇḍalinī is about experiencing Her Bliss, we have to keep our body as well as mind in purest possible conditions. Experiencing Her Bliss is possible only after attaining Her initial Grace. There are two types of Grace, one is Her relative Grace and another is Her Absolute Grace. When relative Grace is compared to Her Absolute Grace, former is the Grace that makes an aspirant to pursue the right kind of spiritual path and this is generally attained by Śaktipāta. If the aspirant pursues Śaktipāta in the way it should, She directly showers Her Grace. The difference between these types of Grace is that the former is done through a Self-realized Guru and the latter is the direct Grace from Her, which is Her Absolute Grace. Relative Grace is the beginning of spiritual path and Absolute Grace is the end of spiritual path (revealing Śiva, which is also known as emancipation).
Ethereal body
Every human has two types of bodies, one is the physical body that we see and another is ethereal body, also known as astral body that is invisible to normal biological eyes, but visible to clairvoyant eyes (Extra Sensory Perception or ESP). Any illness or disease has to first penetrate ethereal body before entering into the biological body. With a little practice, all of us can see our own ethereal body or others ethereal body. Ethereal body will be in the form of smoky light around the body. Luminosity of the ethereal body depends upon the level of energy generated by the physical body, which in turn depends on various factors such as food, thought, breath, etc. Apart from these factors, psychic centres in the body also play a significant role in the luminosity of ethereal body. These psychic centres are situated in the spine, whose energy level is felt both at the anterior and posterior side of the body. These psychic centres are energised on account of two factors and they are kuṇḍalinī energy and the cosmic energy. Cosmic energy is derived from the top of the skull, known as brahmarandhra, also known as the orifice of Brahman. Under normal circumstances, cosmic energy is derived through medulla oblongata and brahmarandhra. These two are not merely energising centres, but according to Ayurveda they also act as marma points (marman means sensitive part of the body or mortal part of the body, that requires to be concealed. Maintaining tuft is based on this principle). 
krikatika marma points
There are two marma points known as “krikatika” situated exactly at medulla oblongata, one on either side of the spine (where spine joins the brain). These two marma points control blood circulation to the head and ensure that the skull is properly lubricated. In addition to this, they also control subconscious mind. The marma point at the top of the head is known as “adhipathi” (adhipa means commander). This marma point controls very important organs connected to spirituality and they are sahasrāra (crown chakra, where brahmrandhra is situated), pineal gland (the gland of Divinity, situated just behind ājñācakra), the entire nervous system and prāṇa (important prāṇa-s such as prāṇa, apāna, vyāna, udāna, and samāna) and is the cause for transcending māyā.  Both these places allow entry of energy from cosmos into the physical body through ethereal body. First ethereal body is energised, which in turn energises the physical body.
These two marma points allow the flow of cosmic energy into the body and by properly directing it towards the resting place of kuṇḍalinī, activates it. When consciousness (awareness) is fixed at the point, where kuṇḍalinī is situated, cosmic energy entering into the body and prāṇa act together to wake up the sleeping kuṇḍalinī from the deep slumber and activates it. Therefore, it is important to know the exact position, where kuṇḍalinī rests. Even under normal conditions, kuṇḍalinī’s slumber is not the same for everybody. The slumber of kuṇḍalinī depends upon one’s spiritual level. Kuṇḍalinī will be in the state of deep quiescence for those who are not spiritually inclined, who do not observe any rituals, who never think about God, who are not compassionate, etc. Waking up their kuṇḍalinī will never be easy and those who do not have Divine bent of mind, should never attempt to practice kuṇḍalinī meditation, as this could lead to serious nervous problems.
 
Kuṇḍalinī in Her dormant state rests at the base chakra, known as mūlādhāra cakra. Lower end of the spinal cord ends at coccyx, which is also known as tail bone. Coccyx, though a muscular area where the spinal cord ends, it has also numerous tendons (connecting muscles and bones; acting like adhesive). Here, there is an inverted triangle within which kuṇḍalinī rests. She is coiled three and a half times around a bindu (this bindu is also known as liṅga). The triangle referred above has a very minute aperture at its lower tip. The tip of this triangle faces downwards under normal conditions. As long as this triangle is facing downwards, one’s sexual energy is in active state. By meditative techniques, which include āsana-s, prāṇāyāma, mudra-s and bandha-s, this triangle can be turned upwards. Once this triangle is turned upwards, kuṇḍalinī which is in the state of deep slumber, begins it’s ascend through citriṇi nāḍi, which is the inner most nāḍi in suṣumna. This is because, once it is turned upwards, the aperture is connected to citriṇi nāḍi directly, making the energy to ascend. This triangle is not merely a triangle. Each of its three sides is represented by icchā śkati, jñāna śkati and kriyā śkati (will, knowledge and action). These three śkati-s are Divine Powers from which, world process begins. When the worldly process begins at mūlādhāra cakra, the lower most psychic centre, liberation happens at sahasrāra at the top of the head. In other words, one spiritual life begins when the triangle at mūlādhāra cakra is turned upwards and ends at sahasrāra (where the union of Śiva and Śakti takes place), where the practitioner is liberated forever. During the initial stages of awakening kuṇḍalinī, there will be too much of secretion of sex hormones and other endocrine hormones are also produced in huge quantities. This results in partial activation of other psychic centres. One has to be extremely careful during the period of awakening, which could last from a minute to perhaps a week or so, depending upon the intensity of practice. Physical body will produce immense heat during this period. One has to excise extreme caution during the period of awakening.

PRACTICE:

Further practice in Nāḍi Śodhana Prāṇāyāma:

Up to stage VI has been discussed in the previous articles. As already discussed, there are twelve stages in nāḍi śodhana prāṇāyāma. All the six stages can be mastered in a matter of two months. It is easier to practice the first six stages and it is going to be difficult to practice the next six stages, as the holding period and exhalation period is more here. Holding period mentioned here are only indicatory and one should not hold breath beyond one’s capacity. Holding breath beyond one’s enduring capacity will even cause death.
In two months, we have 60 days and the chart below will explain how all the twelve stages can be practiced in 60 days. This chart is only indicatory in nature.
Stage
inhalation
retention
exhalation
ratio
duration
I
slow
nil
slow
NA
3 days
II
slow
nil
slow
1:0:1
3 days
III
5
2
5
1:0.5:1
3 days
IV
5
5
5
1:1:1
3 days
V
2
8
4
1:4: 2
3 days
VI
4
16
8
1:4:2
3 days
VII
6
24
12
1:4:2
4 days
VIII
8
32
16
1:4:2
5 days
IX
10
40
20
1:4:2
6 days
X
12
48
24
1:4:2
8 days
XI
14
56
28
1:4:2
9 days
XII
16
64
32
1:4:2
10 days
TOTAL
 
 
 
 
60 days
 
This schedule is for a period of sixty days. One can make minor adjustments in the number of days. In particular, Stage VII to XII should be practiced very carefully. One should not hold breath beyond one’s capacity. Once stage XII is reached, one should continue to practice this forever. Further, unless perfection is attained in a particular stage, next stage should not be practiced. It is also not mandatory that one should follow this 60 day schedule and can comfortably be extended to suit one’s convenience. From stage VII, one can allow sufficient gap between two rounds. Numbers in inhalation, retention and exhalation are counts. One can count mentally. The basic principle is that inhalation, retention and exhalation should be in the ratio of 1:4:2. Under any circumstances, retention should not exceed 4 times of inhalation, As far as exhalation is concerned, counts be increased if possible. As a principle, inhalation should be faster and exhalation should be slower.