The majority of submission holds can be grouped into two broad categories: joint locks and chokes. Joint locks typically involve isolating an opponent's limb and creating a lever with the body position which will force the joint to move past its normal range of motion, generally referred to as hyperextension. Pressure is increased in a controlled manner and released if the opponent cannot escape the hold and signals defeat by submitting. Opponents can indicate submission verbally or they can tap out (i.e. tap the opponent, the mat, or even themselves, several times.) A choke hold, disrupting the blood supply to the brain, can cause unconsciousness if the opponent does not submit soon enough.
A less common type of submission hold is a compression lock, where the muscle of an opponent is compressed against a hard, large bone (commonly the shin or wrist), causing significant pain to the opponent. These types of locks are not usually allowed in competition due to the high risk of tearing muscle tissue. This type of lock often also hyper-extends the joint in the opposite direction, pulling it apart.
While many joint locks are permitted, most competitions bar or restrict some or all joint locks involving the knees, ankles, and spine. The reason for this is that the angles of manipulation required to cause pain are nearly the same as those that would cause serious injury. Joint locks that require a twisting motion of the knee (called twisting knee locks or twisting knee bars, or techniques such as heel hooks, and toe holds) are usually banned in competitions because successfully completing the move nearly always results in permanent damage that requires surgery. Similarly, joint manipulations of the spine are typically barred due to the inherent danger of crushing or mis-aligning cervical vertebrae. Certain locks involving the knees and ankles are only allowed in competition starting at the brown belt. Any competitor from white to purple belt who attempts any of those locks may be disqualified.
However, most joint locks involving the wrist, elbow, shoulder or ankle are permitted as there is a great deal more flexibility in those joints and those locks are safe to use under tournament conditions. Also, some fighters practice moves whose sole purpose is to inflict pain upon their opponent, in the hope that they will tap out. This includes driving knuckles into pressure points, holding their opponent's head in order to tire out the neck (called the "can opener" or kubi-hishigi) and putting body weight on top of the sternum, floating ribs, or similarly sensitive bones. These moves are not true submission moves - they are generally only used as distractions mostly in lower levels of competition. They are avoided or aggressively countered in middle to upper levels of competition.
Chokes and strangles
Chokes and strangles (commonly but somewhat incorrectly referred to as "air chokes" and "blood chokes" respectively) are a common form of submission. Chokes involve constriction of the windpipe (causing asphyxia.) Strangles involve constriction of the carotid artery (causing ischemia.)
Air chokes are less efficient than strangles and may result in damage to the opponent's trachea, sometimes even resulting in death. By contrast, blood chokes (strangulations) cut the flow of blood to the opponent's brain, causing a rapid loss of consciousness without damaging any internal structures. Being "choked-out" in this way is relatively safe as long as the choke is released soon enough after unconsciousness, letting blood back into the brain before oxygen deprivation damage begins. However, it should not be practiced unsupervised.
The prevalence of the more dangerous "air" chokes has led to the banning of choke holds from some United States police departments. Because of the negative legal connotations of the words "choke" and "strangulation", it is advisable to use the term "lateral vascular restraint" when describing a blood choke used in a self-defense situation.