When you really concentrate, it involves the entire body-mind. The power you generate with strong concentration can keep you warm, even in the coldest winter weather. So when you sit, please be attentive.
Once you have regulated your posture, take a breath and exhale fully. Swing to the left and right. Sitting fixedly, think of not thinking. How do you think of not thinking? Nonthinking. This is the essential art of zazen.
[...from On Zen Practice - Body, Breath and Mind by Taizan Maezumi & Bernie Glassman, page 34 from the section On Form}