tongue splitting

The tongue splitting procedure is the central bifurcation of the tongue, so as to achieve a "forked tongue." With practice, each half can be separately controlled.

Placement

The median fibrous septum that centrally divides the tongue is cut, separating the two lateral halves of the tongue. Done carefully, very little damage should be done in the process.
The tongue can, in theory, be split back to where it meets the base of the mouth. Attempting to split past that would endanger muscles that shouldn't be split, as well as glandular structures in the area. Note again that once the tongue is split further than suggested, there may be some minor speech artifacts.
It is not a good idea to cut the tongue in more than two pieces. To do so would risk cutting into the lingual nerves, the lingual glands, numerous major blood vessels, and it is dubious whether full nervous control could be sustained even if done by an oral surgeon. Attempting a "trifuraction" (or more) would almost certainly end in disaster.

Procedure

Tongue splitting can be accomplished by one or more of the following methods:

Oral surgery

The most recommended method is to seek an oral surgeon. Any oral surgeon who uses a laser should be able to bifurcate a tongue with ease, providing their ethics allow it. A doctor will normally charge between $500 and $3000 for this procedure. After applying anesthesia, using a laser, the practitioner will first create a guideline along the top and bottom of the tongue. Then, they'll slowly cut through the tissue until the tongue is fully bifurcated, cauterizing the wound as they cut. This is usually, at most, a fifteen-minute process. In general, there is little to no blood during this stage. The only bleeding tends to be during suturing.
Laser procedures usually take 1-2 weeks for primary healing, and a month for the healing to be complete.

Scalpelling

To put it simply, the tongue is split down the middle using a blade. This is generally very bloody, and this bleeding can be difficult to control. Many practitioners choose to use cauterization, either from electric cauterizing pens or traditional branding tools, on large blood vessels. Others use chemical or herbal means to control bleeding and/or to assist in healing.
Prior to the split, many practitioners insist on a well-healed large-gauge tongue piercing being in place. If there is no existing piercing of this type, a large-gauge piercing is often made by performing a piercing using a #11 scalpel along the grain of the muscle, followed by a taper and 2ga or 0ga jewelry. That should heal within a month or so, setting a good foundation for the split.
Other practitioners choose to use a "clamp and cut" method similar to the home-meatotomy technique, although it is relatively uncommon. Using a long clamp, the tissue to be split is compressed to almost paper-thickness and left that way for 45 minutes. When the clamp is removed, the cut is made along the center of this compressed tissue in hopes that the extreme compression will provide a "seal," minimizing or eliminating bleeding.

Cauterizing

A cauterized tongue splitting is similar to a scalpelled tongue splitting, but a heated blade or tool is used to burn the split through the tongue, hopefully (and generally successfully), cauterizing the tongue in the process, almost totally eliminating bleeding. This technique does carry a greater risk of peripheral damage and may be outrageously painful.

Suturing

Suturing pulls the top skin of the tongue down into the split, leaving a more "natural" and rounded appearance that many people prefer. While similar healing often occurs without suturing, in other cases a non-sutured tongue may heal with a somewhat "flat" inside that looks artificial rather than natural.

Risks

Assuming the procedure is done by a doctor, the risks are negligible, and complications that arise in a controlled medical environment can typically be dealt with easily. Doctors will provide you with paperwork explaining the secondary risks, such as reactions to anesthesia.
If the procedure is done outside of a controlled medical environment, loss of blood is the single largest risk. The tongue contains major blood vessels, and without proper training and tools, amateur practitioners may find themselves unable to control blood loss. In these situations, a visit to the hospital emergency room is advisable.
Damage to nerves and glands in the tongue is also possible, although unlikely if the split is not extremely deep and remains centered. Care must also be taken not to accidentally damage glands in the base of the mouth.
Infection and scarring are certainly possible as well, but they are very rare.

Healing and aftercare

Tongue splitting takes about as long to heal as tongue piercing does. Primary healing (where you can talk and eat relatively normally) takes between one and two weeks, and full healing is usually complete within one month.
Regrowth is quite common. Some re-growth (where the tongue slowly heals back together) is normal, and in the case of short (1/2" or less) splits, total closure is not unheard of. Even in deeper splits, if care is not taken to combat re-growth, 50% closure is not uncommon, but this can be prevented by keeping the wound open. The smooth body of a cotton swab can prove useful for this task. It should be done regularly as the body heals incredibly fast. In addition to the regrowth that will occur during the initial healing, there will be some closure over the first year or so. Short of re-cutting, there's not a lot that can be done to stop that.
Many people have found that a well-healed large-gauge (4ga - 0ga usually) tongue piercing can act as an anchor for the split. Because the piercing is surrounded by solid tissue that does not, by nature, want to seal itself, if this tongue piercing marks the rear of the tongue split, closure may be all but eliminated.

Long-term health issues

Initial bleeding and infection can be a risk, especially with home-jobs. In a worst-case situation, the tongue can swell and block the airway, although this has not been documented. As far as long-term risk, there may be minor changes in some speech sounds, but this rarely happens, and effects are extremely minor at most. Tongue splitting does not appear to have any side effects with regards to eating.

History

Tongue splitting, to some, a pinnacle of "Khechari Mudra" practices, is a part of Hatha and Kumbhaka yoga where the tongue is split and then "milked" until it is long enough to be turned back inside the mouth and flipped up to the epiglottis. It then is used in breathing exercises, the goal being to seal the body's energy leaks and become aware of only the internal, thus entering a catatonic state, crossing back and forth between life and death (note that Westernized versions often omit, and even censor, the splitting).
Yogis that practice these rites believe that it allows them to be absorbed "into God" (that is, becoming conjoined with the universal soul, an experience that is also common in suspension and other body rites).
In addition, Kaliya and other characters in Hindu mythology are depicted as having split tongues, as are characters (often evil) in various other faiths. Most obviously, Christian mythology bestows a forked tongue upon Satan.
Recently, starting in 1997, tongue splitting became very popular in the West. I Am Not My Body (issue #2, summer 1997) postulated the possibility, and the first person (modern/Western) to be clearly documented as having this procedure was a man in Italy who had it done progressively by his dentist using silver nitrate for cautery. The second is believed to be Dustin, featured in Fakir Musafar's Body Play Magazine (issue 16). Dustin achieved her tongue split using the tying-off method over several weeks. Next is believed to have been The Lizardman and then Shannon Larratt, with their promotion of the modification starting the snowball rolling. Documenting the other "firsts," after that, Patrick Bartholomew did the first cautery (heat) tongue splittings in 1998, followed by Allen Falkner's strike branded split; the first scalpelled tongue splittings are believed to have done by Todd Bertrang and Blair at about the same time (first on himself and then on Philip Barbosa).
Since about 2000, tongue splitting has been one of the most popular, common, safe, and highest long-term satisfaction extreme modifications out there.

Reversal

By removing the skin on the inside of the split and then suturing the tongue back together, it can be induced to "go back to normal." While, to date, no one has gone on record as willingly having undergone such a procedure, it has been forced on some people by new military regulations in the United States. Tongue splitting reversal is far more painful than splitting, takes far longer to heal, and seems more prone to complications.
Because of scarring and other damage done during the reversal operation, permanent damage to tongue mobility is not unheard of, leaving a shortened and narrowed tongue that is not as nimble, which of course damages speech and other activities. In addition, nerve damage from the reversal procedure is more likely than from the initial split, and there have been reports of damage to both touch and taste sensations in tongue splitting reversal procedures.
Reversing a tongue splitting is strongly discouraged—don't split your tongue if you think you might have to reverse it in the future.