If we investigate the atemi waza and kansetsu waza historically and logically we see two characteristics:
1. Controlling an opponent by striking, punching or kicking physiological weak points or by spraining or dislocating joints, ie. techniques (including chokes) where the aim is to kill or inflict injury.
2. Exploiting mechanical weakpoints (theory of breaking balance) by applying force at one point or moving joints to their limit, ie. techniques to restrain an opponent with minimum force. This means throwing or restraining an opponent by taking the opportunity to break his balance by shifting his centre of gravity.
Separately, both of these elements can be used to demonstrate good techniques. However, in competitive judo from an educational viewpoint, we should not cause injury when attacking physiological weakpoints. Our techniques should be restricted to those that attack only mechanical weakpoints, ie. concentrating on nage waza (throwing techniques) and osae waza (pinning techniques). This is why the study of atemi waza and kansetsu waza is slow when compared to nage waza and osae waza. Other techniques are not important or are excluded completely.
In the many old styles of jujitsu it appears that atemi waza and kansetsu waza were important. So, if we study these techniques thoroughly we will see their essence, ie. the use of the body and coordination of power, in today's training.
In atemi waza striking at the physiological weakpoints generally causes a loss of consciousness while kansetsu waza generally break joints. This is one aspect of course but if we pursue the deep underlying principles the injuries caused are incidental. The main purpose is to study and understand these principles and use them to throw or pin an opponent with good techniques.
The atemi waza in aikido are used to break an opponent's balance, the aim is not to inflict injury with the impact of a strike punch or kick. Consequently, there is no need for any special training to toughen the hands, fists, etc.
The nage waza in judo use the feet and hips to break the balance of an opponent and throw him. They mainly use the application of two forces at two points, ie. a couple. Similarly, the atemi waza use the hands or arms to push or throw an opponent with the application of one force at one point but the principles of balance breaking are the same.
Joint techniques in judo are mainly elbow techniques. In aikido, the hiji waza (elbow techniques) and tekubi waza (wrist techniques) of the kansetsu waza are particularly good. Kansetsu waza are split into the following two groups:
A) nage waza (throwing techniques)
B) osae waza (pinning techniques)
In the nage waza in A, to break an opponent's balance the joints are controlled at the limits of their movements. This is the same principle as in the nage waza of judo. Also, in the osae waza in B, the aim is achieved by controlling the moveable points. This is the same principle as in the osae waza of judo.
In competitive aikido, the characteristics of 1. are prohibited and the characteristics of 2. are adopted. So, we can have matches of the form 'empty hand against tanto' according to particular rules. In this way the atemi waza and kansetsu waza, which are the important parts of the old styles of jujitsu, are used in a new budo that is a modern form of physical education.