All mantra-s are considered secretive in nature and Ṣodaśī is not an exception.  Recitation of Ṣodaśī leads to liberation.  This mantra does not give any materialistic gains.  It leads straight to the Brahman.  Normally, one is not initiated into this mantra straight away.  Guru decides the timing of initiation into this mantra.  Generally one is first initiated into Bālā.  Depending upon one’s progress, Pañcadaśī is initiated.  If Guru considers that his disciple is fit for final liberation, he initiates him into Ṣodaśī. One should recite 900,000 times of thismantra followed by puraścaraṇa rituals to attain siddhi. Puraścaraṇa is only for 100,000 recitations. Then only liberation is possible. It is also said that initiation into Ṣodaśī depends upon one’s karmic account.

Ṣodaśī vidyā is considered as Brahma vidyā, knowledge of the Brahman.  Brahman is revealed in the form ofmantra-s in Ṣodaśī vidyā. Since Ṣodaśī reveals the Brahman in the form of mantra-s, it is treated as highly secretive in nature.  But the important aspect of its secrecy is the replacement of second Om in this mantra with the aspirant’s ātma bīja.  The third aspect of the secrecy is the worship of ninth āvaraṇa of Śrī Cakrā that deals with parā, parāpara and apara states (these states have been dealt with while discussing nāma-s).  If one is able to reach the fourth stage of turya or turīya, he gets prepared to attain liberation in the next stage ofturyātīta.  Turya is reached without any difficulty when Ṣodaśī mantra is mentally recited regularly.

(Turya and turyātīta: Turya is the fourth state of consciousness, the other three being active, dream and deep sleep. Turya stage transcends all the above three stages by bundling them out. The level of consciousness atturya stage is very close to the stage of blissfulness, derived from experiment and observation rather than theory. Turyātīta is the stage where one’s consciousness transcends turya stage.  In this stage of blissful consciousness, the Brahman is realised where one feels that “I am That” or aham brahmāsmi. The final stage of merging into the Brahman is kaivalya, when a soul ceases to transmigrate.}

In Pañcadaśī one can transcend the fourth state of consciousness, the turiya state.  In Ṣodaśī one can merge with the Brahman, by reaching the fifth state of consciousness, turyātīta. There is nothing beyond this.  What happens if one transcends turya state? The self is replaced by SELF.  This transformative realization happens in a fraction of a second where near death-like situation is experienced. One is not the same person after that ‘second’.

Ṣoḍaśī mantra is superior to Pañcadaśī.  Ṣoḍaśan means sixteen and ṣoḍaśaḥ means sixteenth.  Ṣoḍaśī mantrais derived by adding one more bīja to Pañcadaśī mantra.  In fact, Ṣoḍaśī mantra consists of twenty eight bīja-sand formed like this. The first Om is excluded for computation, as all the mantras begin with ॐ.  

1. Om ॐ
2. śrīṁ - hrīṁ - klīṁ - aiṁ - sauḥ: श्रीं ह्रीं क्लीं ऐं सौः (5 bīja-s)
3. om - hrīṁ - śrīṁ ॐ ह्रीं श्रीं (3 bīja-s)
4. ka – e - ī – la- hrīṁ क ए ई ल ह्रीं (5 bīja-s)
5. ha - sa – ka – ha - la - hrīṁ ह स क ह ल ह्रीं (6 bīja-s)
6. sa – ka - la - hrīṁ स क ल ह्रीं (4 bīja-s)
7. sauḥ - aiṁ - klīṁ - hrīṁ - śrīṁ सौः ऐं क्लीं ह्रीं श्रीं (5 bīja-s)

If this mantra is observed, one can find lines 4, 5 and 6 are the Pañcadaśī mantra and each line representing one kūṭa of Pañcadaśī.  If lines 2 and 7 are observed, the bīja-s contained in the 2nd line are placed in the 7thline in reversed order.  For example, the last bīja in line two is sauḥ: and this is placed as the first bīja of the 7thline. This is called saṃpuṭikaraṇa or encasing of a mantra so that the power of mantra remains within the practitioner.

This mantra is known as Ṣoḍaśī because of sixteen bīja-s, each kalā (kalā means a part) representing a kalā of moon.  This is arrived at by adding Lakṣmī bīja śrīṁ (श्रीं) at the end of Pañcadaśī mantra.   These sixteen bīja-s are arrived at by considering each kūṭa of Pañcadaśī mantra as one bīja.  This way, lines 4, 5 and 6 are considered as one bīja each, thus forming three bīja-s.  The sixteen bīja-s are arrived by adding 5+3+1+1+1+5 (from line 2 to 7).  There are two ॐ in this mantra.  The first ॐ is not considered for calculation.  The secondॐ in line 3 is replaced by ātma bīja of the practitioner and this is decided by one’s guru.  Such a decision can be made by a guru if he is conversant with mantra-s and bīja-s.  A wrong bīja can destroy the practitioner.

Ṣoḍaśī mantra is meant exclusively for liberation and those who seek liberation alone should be initiated in thismantra.  Ṣoḍaśī mantra is the ultimate of all mantra-s and there is no other mantra superior to this. Those who are initiated into this mantra are not supposed to prostrate before anyone except his guru.  Śaktī is worshipped in ten different forms and this is known as daśa mahā vidyā and ṣoḍaśī is one among them.  Ṣoḍaśī vidyā has too many prescribed rituals.

Like every mantra Ṣoḍaśī mantra also has curse removal mantra, also known as śāpavimocana mantra. All along this mantra is not revealed to all the aspirants. If curses on mantras are not removed, mantras will not fructify. Śāpavimocana mantra for Ṣoḍaśī mantra is given below for the benefit of sincere aspirants. However, one should obtain prior approval from one’s guru. Śāpavimocana mantra, which consists of three parts should be recited before the commencement of mantra japa. First part should be recited seven times, second part three times and the third part one time.

First Part – should be recited seven times:

ई ए क ल ह्रीं  ī e ka la hrīṁ

ह स क ह ल ह्रीं  ha sa ka ha la hrīṁ

स क ल ह्रीं  sa ka la hrīṁ

Second Part – should be recited three times:

ह स क ह स क ह ल ह्रीं ha sa ka ha sa ka ha la hrīṁ

स क ल ह्रीं sa ka la hrīṁ

ई ए क ल ह्रीं ī e ka la hrīṁ

Third Part – should be recited one time.

ह ल भ भ भ भ भ अ ha la bha bha bha bha bha a

Full text of mantra japa is available in this link

Mahāṣoḍaśī mantra is explained in detail here.