This Japanese technique is written using two Chinese and one Japanese ideogram - with all three ideograms being routinely used in the Japanese written language - and two ideograms ('1' and '2') being used in the Chinese written language:
腰 - Japanese - Koshi (Chinese: yao1) = waist, hips and midsection
投 - Japanese - Na (Chinese: tou2) = throw, blend, redirect and reject
げ - Japanese - Ge = down, low, depth and ground
Interestingly, with regards the ideogram '腰' (Koshi) - both the Japanese and Chinese language dictionaries give an identical (and exact) physical location. Therefore, 'Koshi' represents the 'waist' (or the anatomical 'space' between the hips) situated toward the front of the body - whilst the back of the body corresponds 'Koshi' to the 'small of the back' or the 'lumbar' region. Although neither dictionary mentions the centre of gravity of the body - or the 'lower dantian' (both situated three-inches below the naval) it seems clear that such a 'special' area is implied. I think this assumption receives support as 'Koshi' is also used to refer to the 'kidney' area - perhaps slightly higher than the lumbar a 'cold' area significant within traditional Japanese and Chinese medicine.
Although 腰 (yao1) is prevalent within Chinese martial arts (used to counter an opponent through penetrating their technique, blending with it and redirecting it) - 'Koshinage (腰投げ)' is a 'distinct' Japanese physical interpretation not found in China. When the Chinese government took Sō Dōshin (宗道臣) [1911-1980] to a Japanese Court in the early 1970s (an event covered in one of Donn F. Draeger's books on 'Modern Budo') - part of the evidence accepted by this Court that his style of 'Shorinji Kempo' ('Shaolin Gongfu') was NOT of Chinese origin - is that nearly all of its techniques include 'Koshinage' (the BBC chose to omit this Court verdict in its 1980 'Way of Warrior' series). Cooperation of this type is a Japanese cultural development - and is not found in China's traditional arts - even though the concept is present (and used in a different manner). However, I would note that the use of squat-kicks is found in Chinese arts and are used in exactly the same manner as this documentary suggests - although our Hakka style places a great deal on toughening the legs to take continuous impact (similar to Muay Thai fighters in Thailand) and keep effectively moving. The only Karate-Do style I have encountered that has squat-kicks is Goju Ryu.
The reality is that the head and centre mass inner organs need to be protected through a) movement of the entire body away from the potentially damaging blows, and b) the movement of the head, arms and legs so as to deflect, parry and/or block the incoming blows when the centre of mass cannot be adequately moved out the way. In the case of unarmed blows - the muscle, bone structure and inner connective tissue can be 'toughened' through physical fitness and martial arts training. The potential damage of these unarmed blows can be effectively absorbed by the outer physical and near surface structures of the body - this includes the bone structures and soft structures that comprise the entirety of skull including the mouth, teeth and tongue, etc. These areas can be 'damaged' but will heal given time. When dealing with bladed weapons - as these can penetrate the toughened outer layers and do fatal damage to the inner organs - movement of the centre mass is of a greater concern if no self-defensive weapon is held by the victim.
In this case the legs must be well trained for either minute or substantial (instantaneous) distance adjustment. A damaged leg will still have to move by necessity (hence the importance of squat kicks in all their variants). The upper limbs comprising the arms and hands will have to 'deflect' and/or catch or absorb the thrusts of any incoming bladed weapons that cannot be completely avoided by systemic (evasive) movement. This may involve the substantial damage of these body parts which must be conditioned to perform this function whilst carrying-on moving for the duration of the self-defence encounter. These types of injuries can heal (regardless of severity and any potential or long-term damage). An individual will probably survive broken or cut arms and hands - but will not survive damaged or cut inner organs, arteries or veins, etc, as the medical chances are greatly reduced.
The outer limbs can be sacrificed to protect the inner organs and blood supply vessels which lie across the centre line. There are cases of individuals surviving horrific injuries following attacks from blades weapons – even with severe mi-section laceration damaging and exposing the intestines, severe and deep cuts to the neck area and even limbs hacked-off – providing medical care can be found in time. Chances of survival are enhanced if the individual concerned has some medical knowledge and First Aid experience – as this can stem blood loss and prolong the chances of survival. Of course, traditional Chinese martial arts often involve extensive mental toughness regimen – and it is this attribute that can drive a severely injured individual to a) ‘survive’ a deadly encounter (despite being badly hurt), and b) seek-out assistance despite being isolated or far away from medical help.
The footage and graphics contained in the following (Chinese language) video are very interesting when it is understood that the bones are aligned, the bodyweight is dropping into the ground (rooting) and a rebounding force is rising. The mind is calm and expansive whilst the joints, limbs (and sections of the torso) are 'rounded' (concave and convex). When the joints are 'rounded' the ligaments and tendons are relaxed and extended (like a drawn bow-string). Although the muscle are relaxed and released from any unnecessary tension - there is a natural 'torque' (twisting tension) - again, similar to a bow-string that is drawn with just the right amount of tension - and no more.
This is important as most people are far too 'tense' and 'rigid' as a habit. Once the awareness penetrates the centre of the bones and all this is achieved - this advanced understanding spreads all over the body and is held in place by an expanded awareness. It can then be reproduced in any and all Form movements and in any self-defence situations. The bodyweight drops down through the centre of the bones (stimulating the bone marrow) and 'rebounds' up from the ground - also through the centre of the bones - and can be emitted through any part of the body. This power production can then be augmented by the localised speed of a limb or position (angle) of the attacking body-part. A perfectly timed blow can be massively magnified in power by catching an opponent 'running in' onto a punch or kick, etc. Here, in this beautiful Shaolin (Europe) video we can see how the bow is correctly 'drawn':
All this is achieved through holding the '站桩' (Zhan Zhuang) - or 'Standing like a Stake' - ('Holding the Ball') position. Although I focus upon the 'rounded' structure of this exercise - its name traditionally derives from a foundational 'stake' or 'pile' (shaped like a 'rounded' Western telegraph pole) that was driven deep into the ground (or sea-bed) which served as an anchoring (and firm) support for a building. There is 'rootedness' (dropped bodyweight) and there is expanding 'stabilisation' (rebounding and rising) power! This is an analogy worthy of further consideration - as it is this psychological and physical 'roundedness' that makes all this ability possible!
Shifu Adrian Chan-Wyles (b. 1967) - Lineage (Generational) Inheritor of the Ch'an Dao Hakka Gongfu System.