Technique can be used in a variety of situations and from various attacks. There are four parts to a technique. There is the block or parry, the counter, the throw or take-down, and the submit. All four parts should be supported with a strong kiai. Slight modifications to a technique make it applicable to different forms of attack. The majority of strikes used in Ju-Jitsu are designed to stun, not injure. Only after the opponent has been placed in an inferior position is a powerful strike used as a submit. Nerve strikes and pressure points can result in the opponent being stunned, in pain, and in some cases even result in the loss of blood circulation or a numb muscle. These strikes loosen up an attacker, buying more time to execute a technique, impeding any resistance, and forcing an opponent's body into a favorable position for the defender.
The mechanics of Ju-Jitsu are based upon physical law and human physiology. The basic physical laws applicable to Ju-Jitsu are Newton's three laws of motion, torque, and gravitation. Equal and opposite reaction and inertia, along with torque and gravity, allow a Ju-Jitsu practitioner to defeat a stronger and larger opponent without using large amounts of muscle strength. Inertia is used in the sense of giving way to the attacker. Instead of opposing the attacker's momentum, add to it. Opposition means the defender would have to stop the attacker coming form one direction, then use power and strength in order to move the attacker to gain the advantage. By going in the same direction as the attacker's momentum, little force is needed to move the attacker off balance or into the desired technique. Also it takes the attacker time to change its own momentum, critical time the defender can utilize executing a technique.
Equal and opposite reaction is sometimes used alone, for example, when throwing a punch, but it is usually used in conjunction with torque. When a person throws a punch, the whole body is used. As the punch moves forward and the hips shift slightly, the feet push against the floor. By equal and opposite reaction, the floor pushes back. This force, as it is transmitted through the body, is ultimately transferred to the punch, adding to the upper body force. When used with torque, equal and opposite reaction can be very effective. Plus there is gravity, which accelerates the attacker. It is the sudden stop when the attacker hits the ground that does the damage.
Human physiology is used in Ju-Jitsu in the form of kinesthetic, pain, and reflex arcs. Humans react in certain ways to pain or discomfort. By applying pain in certain ways, the defender can cause the attacker to move itself in a desired way. One of the most common methods of accomplishing this is to move the attacker's head. A human does not like the idea of its head being dislocated from its body, so when the head is moved, a human will move to compensate. In addition, the body can only move in certain directions. Locks take advantage of this fact, often along with nerve techniques to help control an aggressor.