The one point (also hara or lower dantien) is often explained as the centre of gravity and this is a good start, or “in the ball park” as a Japanese Sensei of mine once joked. But this is only the stepping off point.
For each of us the One Point is the centre of the universe.
The universe is of infinite size and therefore every point in it is also at its centre. Imagine the universe shrinking around you until it is compressed to a single point as inconceivably small as the universe is inconceivably large. This is the One Point, and we can learn to control and use it.
I have found that the One Point becomes the centre of my mind and can be moved about to stabilize myself mentally or physically.
In the West there is a saying “to keep yourself grounded” and this too is a good place to begin, as it too incorporates control of both your physical body and your mind.
To keep one point is the first of the four basic principles of Aikido (Ki no fudo ho). If we achieve one of these principals, we also achieve the others but if we try to achieve more than one we will not achieve any.
It is as easy to recognise when someone has one point; they are relaxed, happy, centred, balanced, “grounded” and they move without breaking this form, they strike the ball sweetly, throw faster, run effortlessly and always have a light floating feeling. The opposite is also true when we lose one point; our form is bad we bounce around and look to be trying too hard.
If your find you have lost one point, shrink the universe by half and by half until it forms at your centre, and let your mind move there where it too is the centre of the universe.