Koshi is the Japanese word for hip and is a very important part of the body. Koshi is the connection between your lower and upper body, which means that correct use of Koshi ensures being able to use the whole body. The first thing that you learn in a Karate Dojo and that which you will hear until the end of days is that you have to twist your hip. Apart from the fact that you have to twist Koshi and that Koshi can twist, for me personally, using the word 'twist' is a matter of convenience.
If you want to keep it simple, you can say that you can twist Koshi (Gyakuzuki / Mawashigeri). You can distinguish two different kinds, twist 'in' and 'out'. The quite good Dutch word for this doesn't translate very well directliy. Google translate tells me that twist in means screw and twist out means clip off. I don't think that helps, but add the info anyway since I can only guess what it means to a native speaker.
Twist in (screw): Twist Koshi in the direction of your front foot (Gyakuzuki)
Twist out (clip off): Twist Koshi is the opposite direction of your front foot (Nagashizuki)
If you carefully study Koshi's movement (and the closely surrounding area), twisting is quite a superficial explanation for using your hip. The dynamic is quite complex if you are taking movement of surrounding muscles, ligaments and tendons into account, which I will address in several blogs. In this blog, I would like to introduce you to using your hip socket, also called Acetabulum in medical terms and Kankotsukyuu in Japanese.
Everybody has a hand and fingers, but to use them in the correct manner for complex and subtle movements requires a lot of practice. The same counts for your hip. To develop Koshi there are several different trainingmethods. In this blog, I would like to share a trainingmethod that I developed at the end of 2012 and have been practicing daily since. This is a method is a combination of a training that saw in a Sumo Dojo when I was in Japan and a method of Nukina sensei, which makes sure that you have to rely on a good Seichusen. In the video, I explain what to pay attention to and I demonstrate the method as well.