‘Continuously I suggest that you have good posture, because posture is a sort of proof of the mind situation, a reflection of your invisible life which penetrates the body. Let your mind ride on good breathing, smooth, deep, even breathing, coming in and going out, which keeps you from slipping out of the present moment. As soon as you forget the breath, mind phenomena color your breath and all sorts of movies go on in that breath. Your body continuously reflects whatever is going on in those personal movies.’ (Embracing Mind)
This is a wonderful subtle observation of the sitting process.