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.
Shifu Adrian Chan-Wyles (b. 1967) - Lineage (Generational) Inheritor of the Ch'an Dao Hakka Gongfu System.