Kechari Mudra is practiced by placing the tongue in the nasal cavity. The gold line symbolizes the nadi going through the center of the tongue. Kechari mudra is achieved by practicing talavya kriya. Students are advised against cutting the frenulum. Kechari can be achieved through dedicated practice of talavya kriya which brings about kechari naturally. Energy radiates through the tongue to stimulate the Ajna center and the 3rd ventricle of the brain. This kriya is taught in the original kriya of Babaji in the Lahiri Mahasaya Lineage. Some organizations omit this kriya, and Nabhi kriya in their instruction of 1st Kriya to their students. However, kechari mudra must be achieved for the higher kriyas taught in the Lahiri Mahasaya Lineage to be effective. Without the practice of talavya kriya or one similar to it, kechari mudra can not be achieved in a natural manner.
Some misguided practitioners of kriya yoga feel that kechari mudra is an unnecessary practice. But they are mistaken in their thinking. Even enlightened masters outside of the kriya yoga lineages have acknowledged that there is a gap between the top of the spine and the medulla oblongata (also known as the alta major center) which connects the spine with the brain. The Tibetan Master DK has stated the following:
"When the fires of matter have passed (united) still further along the etheric spinal channel they contact the fire of manas as it radiates from the throat center. ...................The other fire of matter (the dual fire) is attracted upward, and merges with the fire of mind through a junction effected at the alta major center. This center is situated at the base of the skull, and there is a slight gap between this center and the point at which the fires of matter issue from the spinal channel. Part of the work the man who is developing thought power has to do, is to build a temporary channel in etheric matter to bridge the gap. This channel is the reflection in physical matter of the antahkarana that the Ego has to build in order to bridge the gap between the lower and higher mental, between the causal vehicle on the third subplane of the mental plane, and the manasic permanent atom on the first subplane. This is the work that all advanced thinkers are unconsciously doing now. When the gap is  completely bridged, man's body becomes coordinated with the mental body and the fires of mind and of matter are blended." Cosmic Fire, p. 138
The practice of kechari mudra bridges that gap. And it has other functions as well.
Some western occultists have also discovered the benefit of kechari mudra. Frater DPAL, a member of a western occult school made the following comment on kechari mudra:
"The Kechari Mudra is the "king of all yogic mudras." This is done thusly---Take the tongue and move it up to the roof of the mouth and back until it slips behind the soft palate. Push it further as it slips behind the nasopharynx. If you can accomplish this feat you can close off either nostril with the tongue from the inside!! As you push on up with the tip of the tongue, you will reach a bone protrusion that is just under the pituitary gland. The force which is rising upward flows off the end of the tongue, like flames under a pot on the stove. It is here that the final connection is made. By exciting the pituitary gland, the pineal
gland begins to vibrate, and the two forces are magnetically drawn to unite in the region of the Spiritual eye. This exercise requires some practice as it is not easy, but lights are experienced and a one-ness is felt---Ananda! "
Editor Comment on Frater DPAL kechari mudra quote: A couple of points may shed light on the above quote. First, it is important to realize that the physical body consists of two parts, the gross physical body and its underlying energy body called by some the etheric double, vital body or pranamaya kosha. The energy body or pranamaya kosha is the principal and determinative component of the physical body. The gross physical body is an automaton that merely reflects in physical form the changes that have occurred in its energy body (pranamaya kosha). The vital body can be changed by thought. Most readers are familiar with the phrase energy follows thought. It is this principle that makes kriya, chi quong, acupuncture, Reiki, pranic healing, etc. effective. Eventually, when the medical world can measure the energy body, it will revolutionize medicine. The kriya yogi focuses his attention entirely in the pranamaya kosha and its network of channels known as nadis. The nadis are the channels in the energy body through which prana flows. Where 21 of these channels intersect, one of the major 7 chakras are found. Since the 7 glands are physical correspondents to the 7 chakras, when the kriya yogi, focused in the pranamaya kosha stimulates a chakra, the physical gland corresponding to that chakra will automatically be stimulated. In the case of kechari mudra, the energy flowing through the nadi of the tongue is directly stimulating the Ajna chakra and this stimulation is reflected in increased activity of its corresponding gland, the pituitary. The important point that I want to make is that at no time does the kriya yogi focus on the glands of the gross physical body, his focus is in the energy body and its chakras.
Now, the fact that the pituitary gland is stimulated has vast implications for the rejuvenation of the body. One of the secrets of the Gorakhnath yogi's, who emphasized the importance of the practice of kechari mudra, was kaya siddhi, extreme physical longevity. The pituitary gland is called the "master gland" of the body because it controls the secretion of hormones. These substances have a dramatic and broad range of effects on metabolism, growth and maturation, sexuality and reproduction and other important bodily functions. As we age, we secrete less hormones. In the elderly, human growth human is reduced to trace amounts. Perhaps the Gorakhnath yogi's ability to rejuvenate the body and extend longevity lies in their ability to stimulate the pituitary gland through the practice of kechari mudra. Today, many older people are attempting to rejuvenate the body by taking supplements to stimulate the secretion of human growth hormone or are using human growth hormone replacement therapy.
There is also another aspect to kechari mudra. We have been taught as above, so below. If dramatic changes can occur in the physical body as a result of the stimulation of the pituitary gland through kechari mudra, then you can be sure that equally dramatic changes are also occurring in the pranamaya kosha (subtle body) and the circulation of the pranas because the physical body only reflects changes in the subtle body. In fact, one the benefits of practicing kechari mudra is that the mind is quieted and it is easier to concentrate. In short, kechari mudra causes changes in the flow of prana, your consciousness, and your physical body. Through its practice, one may experience the elixir of life, amrita.
Listed below are the specific hormones produced by the pituitary:
Thyroid Stimulating Hormone (TSH) - As the name implies, TSH stimulates the thyroid gland to release thyroid hormones. Thyroid hormones control basal metabolic rate and play an important role in growth and maturation. Thyroid hormones affect almost every organ in the body.
Growth Hormone (GH) - This is the principal hormone that, among many other functions, regulates growth and metabolism.
Adrenocorticotropic Hormone (ACTH) - ACTH triggers the adrenals to release the hormone cortisol. This hormone, in turn, regulates carbohydrate, fat, and protein metabolism.
Antidiuretic Hormone (ADH), which increases reabsorbtion of water into the blood by the kidneys and therefore decreases urine production.
Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH) - These hormones control the production of sex hormones (estrogen and testosterone) as well as sperm and egg maturation and release.
Melanocyte-Stimulating Hormone (MSH) - Controls darkening of the skin.
Oxytocin - Stimulates contractions of the uterus during labor and the ejection of milk during breast-feeding.
Prolactin (PRL) - This hormone stimulates secretion of breast milk.
Vasopressin - Also called anti-diuretic hormone (ADH)- This hormone serves to allow the water to be reabsorbed by the kidneys.
Kechari Mudra is discussed in Chapter 3, verses 32 thru 53 of Hatha Yoga Pradipika. Shri Shailendra demonstrates kechari mudra at http://www.siddhasiddhanta.com/khechari.html An informative discussion on the stages of kechari mudra can be found at http://www.aypsite.com/108.html Also see the section on Kechari Mudra and the section on the "Knots" in Kriya Quotes from Swami Satyananda located at http://www.yoganiketan.net/main.htm in the kriya library