Revealing the Truth, imparting the pure knowledge of Truth, transmitting the spiritual energy, purifying the disciple by burning his karmas or sins through different rites and disciplines--all this is included in the word "diksha" (initiation).
Diksha dispels the ignorance of the disciple, removes the impurities in his body and mind, and blesses the disciple with the fragrance of holiness. Wisdom, Light and inspiration energy is transmitted into his heart and the disciple experiences divine joy and peace.
There are three kinds of higher initiations. They are called shakti diksha, mantra diksha, and shambhavi diksha. When the guru awakens the shakti kundalini (the divine energy located at the base of the spine) and makes it flow through the spiritual spinal column (sushumna nadi) until the shakti merges in Shiva (pure consciousness of God), it is called the "shakti diksha".
When the disciple has prepared himself with fasting, prayer, and other disciplines, seeing the place and the time as auspicious, the guru expands the disciple's consciousness by imparting a power-word or mantra. This is called "mantra diksha." And when the guru is in a high divine mood and feels that the disciple is ready and worthy of such a high initiation, he leads him to the pinnacle of divine experience either by a glance or looking into his eyes or by a touch--making him one like himself--then such a kriya or transmission of energy is called "shambhavi diksha".
All have to go through the mantra diksha to receive the shakti diksha or shambhavi diksha. Either by the merits of past lives or through rigorous spiritual disciplines of this life, or by a special grace of God or Guru, one is blessed with these initiations. These three initiations are known as maha-diksha or great initiations.
Now, there are four smaller initiations to prepare the disciple for the higher initiations. They are called Kriyaavati, Varnamayi, Kalaavati, and Vedhamayi.
Kriyaavati initiation is related to several kinds of rituals and worship services that purify the disciple by bringing the blessings of deities. Varnamayi initiation is called nyaasa, which means locating the consciousness in different parts of the body and limbs and making the mystical syllable vibrate that part of the body until the disciple feels the throb of the mantra all over his body.
Kaalavati shakti initiation is extraordinary. Through this initiation, the guru integrates the five kinds of shaktis in the body of the disciple together and makes the disciple realize his great Self. These five different shaktis are known as: nivritti shakti or the energy of detachment, which flows from the soles of the feet up to the knee; pratista shakti, or the settled energy that flows from the knee up to the navel center; vidya shakti, or the wisdom energy that flows from the navel center up to the throat chakra, shanti shakti, or the peace energy that flows from the throat center to the forehead center; shaantyaatitakalaashakti, or the energy of the "peace that passeth all understanding" that flows from the forehead to the top of the head or crown chakra. In this initiation, the spiritual Master calls all the shaktis from the body of the disciple by the power of the Lord and integrates them all in his crown chakra and gives him an experience with Lord Shiva.
The fourth initiation is known as Vedhamayi, which means making the energy kundalini to pass through all six chakras or mystic centers in the astral body. The guru does this with his grace upon the deserving disciple by uttering the word or the mantra for each chakra and leading him gently through the sushumna to the crown center.
There are two other kinds of special dikshas known as panchaayatanee diksha and krama diksha.
In the panchaayatanee diksha, five aspects of God known as Shakti, Vishnu, Shiva, Surya and Ganesha are invoked in the body of the disciple. In the outward worship, any one of these aspects of God is given importance as per one's chosen deity (Ishta Devata), and the other four are kept in the four directions, keeping the chosen deity in the middle. Then the puja (worship) of the deities is conducted by the disciple as per the instruction of the guru. Thereafter, the guru will explain the mystical meaning of that worship and invoke those deities in the body of the disciple and give him an experience of the Divine. By this experience, he is able to see many in one and one in many.
Krama diksha means an initiation in which the disciple will have a gradual expansion of consciousness. Service and obedience to the guru plays an important role in all the initiations and especially in this diksha. Daily participation in worship of God, chanting of the mantras and prayers, listening attentively to the words of the guru, reverently bowing before him again and again with devotion, serving him selflessly, and pleasing him through seriously practicing the disciplines given by him, are some of the ways through which the disciple prepares himself to receive the light.
After the initiation, guru will give some instructions which should be carried out by the disciple. Some such instructions are as follows:
1. The disciple should not tell the guru-given mantra to anyone. He should not say the mantra publicly.
2. If the guru has taught a private puja or worship of a deity as a sadhana or spiritual discipline, it should not be revealed, taught or publicized unless through the guru's permission.
3. Holy scriptures like Vedas, Upanishads, Bhagavad Gita, etc., should be read. And whichever scripture the guru has recommended to be read, the same should be read with all devotion.
4. Honoring other Masters, serving the needy and the poor, serving the aged parents--all kind acts should be done diligently.
5. Rhythmic breathing, japa (repetition of the mantra), puja, and meditation should be practiced everyday without fail.
6. Any great spiritual experience or dream experiences should not be said before others, other than one's own guru.
7. One who scrupulously follows the spiritual instructions of the guru certainly attains the Supreme.
In the Kulaarnava Tantra, three more kinds of dikshas are explained. They are sparsha diksha, or initiation by a touch; drik diksha, or initiation through a glance; dhyaana diksha, or initiation through the guru's mere thought of the disciple. Sparsha diksha occurs when the guru, attuning himself with God and preparing the heart of the disciple, touches the disciple and transmits the spiritual energy. Most of the time in this kind of initiation, the guru places his right palm filled with the grace and the spirit of God upon the head of the disciple. Paramahamsa Sri Ramakrishna gave this touch initiation to Naren (later known as Swami Vivekananda) by placing his foot on Naren's thigh. A tornado of light permeated Naren's being, which tossed him into samadhi (the superconscious state).
After a long meditation, as per the will of God, when the guru looks at a disciple who has prepared himself and is very eager to receive the blessings, a flood of energy of light and love flows from the eyes of the guru into the eyes of the disciple and the heart of the disciple fills with ineffable joy and peace, it is called drik diksha. When Sri Ramana Maharshi, the sage of Arunachala, looked into the eyes of Paul Brunton, Brunton had a blissful experience with the Atman, the great Self. Ancient Masters of the Himalayas used to give such initiation to the deserving disciples.
Initiation given just by a powerful thought to the disciple in meditation by the guru is a unique kind of initiation. The disciple may be living thousands of miles away, but the guru can initiate him by giving a great spiritual experience just by a powerful thought. Sometimes the guru may write to the disciple to prepare himself and sit in the lotus posture at a fixed time indicated by the guru. Sometimes, the guru may even enter into the dream of the disciple and impart the mantra to bring the awakening in the life of the disciple.
(Sadguru Dattatreya, p. 51.)