Depending on each person, there is an inner image of what breathing in sitting is. As you notice, there is also a physical element of sitting and an invisible element of sitting, which we call mind. We do mind-sitting, body-sitting, and we let the breath sit.
Three aspects to sitting exist because we can observe our sitting from three angles. We breathe naturally and appreciate our breath and really understand what the breath does to our body and mind. To really connect the three—body, mind, and breath—is the point.
During sitting, your breath should be very regular, very smooth, with almost no effort, not noticing that the air is gone, or has come in. Breath has an incredible range of volume, strength and speed. There are hundreds of techniques you can use, depending on your health and emotional condition. Like playing an instrument, singing, or drawing, you breathe; there are many ways. The basic point is not to push or pull, but to let it go.
The ancient Sanskrit word for breath was prana. This is translated as chi in Chinese or ki in Japanese—ki, as in aikido. Ki is vitality. Sometimes it is called seiki: life-vitality. And this soft part where our intestines are is called hara in Japanese. Hara is also called ikai: the ocean of ki. Our vitals are here. When you have no strength in the hara you feel very weak. When you are full of energy this part is full of energy. When you chant you let your voice come out from this center of your stomach. Basically ki comes out and informs the shape of your mind. The contents of your mind is that voice.
The ideal in sitting is to forget the breath. You may breathe as you like; there is an incredible variety in the speed of breathing, and even the emotion of breathing. So if you want to observe your breathing, you should do it for months and months without trying to control it.
My feeling is that each breath is an independent thing. It arises and goes and some thoughts go with it. You cannot bring them back. That's it. It's the same as your heartbeat—your whole body is needing it. So if you can forget the breath then you are having perfect breath. I suggest that you keep your posture straight, upright—good posture, that naturally takes care of the breath.
From deep breath, which carries your awareness with it, to very shallow breath, which also carries your awareness, you have to choose the best breath between them. You can be aware of the texture of your breath, from rocky breath to silk-like breath, and finally to transparent breath, like a transparent string of breath. You can feel which is the best breath for your sitting.
Try to sit and pay attention to how your breath goes. Each time you sit, your body condition is different, so each time you must try to find your best breath and stay with that. Get really familiar with it. I always feel breathing is like drawing a circle.