Literally this means the back of the hip. Physically, techniques done using the hips are part of Aikido, the same as in Judo. Spiritually or Ki-wise it has a much more significant meaning. As the true meaning is so misunderstood as well as being hard to put into words I shall attempt to explain it in stages.
Firstly, the condition of weight underside comes about through using the principle of letting go.
Letting go of means; spiritually letting go of. If you sit quietly and extend your arms out horizontally and hold them there, you will find the effort of doing so makes the muscles and the shoulders get tense. If you focus on the place that gets tense and spiritually let go of that part of the body you will find it relaxes. In fact, if you focus on any part of the body that's tense and spiritually let go of it you will find it relaxes. This is fascinating in itself.
If you remain focused you will notice that as that part of the body relaxes the energy seems to go downwards. Thus you achieve weight underside.
This shows you that tension is you holding on spiritually, maybe you think that if you let go you will lose something, or it will run away or collapse. Actually, by spiritually letting go you are allowing your Ki to flow through naturally and thus the body feels better and also relaxed.
Another fascinating thing about weight underside using the whole body, is that you would think it makes you stick to the floor, well it makes it impossible for you to be lifted up but at the same time you can move about quite comfortably and with ease. That is weight underside.
What is Koshi?
Koshi is letting go of everything, meaning you are joining as one with your opponent without spiritually holding onto them or to yourself. This is experiencing without attachment yet at the same time being with and in harmony. This needs practice in order to get the reality of it, and is different to the fact of being able to keep center and keep space.
So if you keep center and practice letting go at the same time, then the more you will feel weight underside. You will eventually get a powerful feeling, your center will feel more powerful, your body will feel more relaxed and then you will start feeling Koshi. You will feel at the base of your spine a space rather than a tense, tight door. This is koshi.
AND IF YOU TAKE IT TO ITS FULFILLMENT YOU WILL BE AT ONE WITH.
Putting this into perspective; once a jockey is comfortable riding a horse, then he/she is at one with the horse, relaxed and apart from being centered he has koshi. Like glue he is connected to the horse yet unlike glue he/she is free to move rhythmically.
HE IS IN THE ZONE!
Here's an exercise for you to do, which will show you the truth of what causes Koshi:
Imagine an opponent in front of you and you cut him down with a sword, feel your energy and the effect of your energy on yourself. You will feel energy going weight upper-side and condensing in tightness and the base of your spine becoming 'closed' and tight.
Now imagine your opponent in front of you and cut through his space with a sword of kindness leaving him/her there, restored, a changed man/woman. This is a different feeling, quite the opposite in fact. In fact you can do this exercise with anything, even a mouse.
Create a mouse and imagine doing something mean to the mouse, then create a mouse and imagine doing something kind for the mouse.
WHEN YOU KNOW WHAT YOU ARE DOING IS KIND, GOOD AND HELPFUL THEN YOU EXPERIENCE KOSHI.
WHEN YOU KNOW WHAT YOU ARE DOING IS OTHERWISE, THEN YOU EXPERIENCE RESISTANCE AND ALL THAT GOES WITH IT.
KINDNESS IS KOSHI AND THE RELEASE OF KI.