Koshi is the Japanese word for hip. It is common knowlegde that you should twist your hip when you execute techniques.
Hip twisting creates power and does so in a large movement. Later on, this kind of movement must be made smaller and smaller. Eventually it comes down to using your whole body effectively— hip twisting may actually be slow. Practically less effective for different situations.
You have to start somewhere— that somewhere is here. By twisting your hip. Learning to control it. Twisting, thrusting, cutting.
I will address all different kinds of uses of Koshi and share what I believe are the most effective methods to develop this.
Learn how to use your hips properly (twisting is just a small part of it). Use Seiza and these Idori movement methods that I instructed at my Idori Seminar 29 June 2019.
Koshi is translated as back, lower back, waist, hips and lumbar region. This is a very important and strong part of the body. Your hip is what connects your upper and lower body and is basically the centre of gravity.
"The upper body should be sitting on the lower body"
That's what Ishikawa sensei said.
Do you get it? I didn't. Not at the time. It was cool to know but it didn't help me.
Johan Cruijff explains this clearly "you only get it when you understand".
Here's the reason: It's a feeling. It's not information - it's knowledge.
It's the description of a feeling. When your whole body is connected. One unit that moves. Produces power. Directs energy. Stands stable. This is the meaning of "Upper body should be sitting on the lower body". It is the connection between the upper and lower body. Once they become one unit, you can feel it. You understand the meaning of 'sitting on'.
Here's a more practical explanation of how to get the feeling. A condition that has to be met >>
Strictly speaking it is only Internal Rotation for the left hip - twisting towards the front leg.
Twist your hip in the direction of your front foot, from Hanmi to Shomen.
Technically, Internal Rotation is used for Gyakuzuki.
Strictly speaking it is only External Rotation for the left hip - twisting away from the front leg.
Twist your hip from Hanmi to Mahanmi.
Technically, External Rotation is used for Nagashizuki.
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 directly.
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.
First, I 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 training methods.
I would like to share a training method 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.