This is what we found on Wikipedia regarding the Armlock:

An armlock in grappling is a single or double joint lock that hyperextends, hyperflexes or hyperrotates the elbow joint or shoulder joint. An armpit lock can be useful; it will immobilize an opponent and pin them on the ground. An armlock that hyperextends the elbow is known as an armbar, and it includes the traditional armbar (pressing the elbow against the thigh or hips), the shoulder triangle armbar (where a figure-four is locked with the legs), and the shotgun armbar (where the opponent's wrist is placed in the armpit, using the forearm as a fulcrum). An armlock that hyper-rotates the arm is known as an armcoil, and includes the Americana, kimura, and omaplata. Depending on the joint flexibility of a person, armcoils can either hyperrotate only the shoulder joint, only the elbow joint, or both the elbow joint and shoulder joint.

Obtaining an armlock requires effective use of full-body leverage in order to initiate and secure a lock on the targeted arm, while preventing the opponent from escaping the lock. Therefore, performing an armlock is less problematic on the ground, from positions such as the mount, side control, or guard. Armlocks are more difficult to perform when both combatants are standing up, though the stand-up variants are a focus in certain systems such as Chin Na.

Armlocks, considered less dangerous techniques in combat sports allowing joint locks, are the most common joint locks used as submission holds. In training, the method of executing an armlock is generally slow and controlled to give the opponent time to submit prior to any infliction of injury. However, in self-defense applications, or when applied improperly or with excessive force, armlocks can cause muscle, tendon and ligament damage, even dislocation, or bone fractures.