It will help to empty the mind of its tumult and the nerves of their agitation if he will breathe out as fully as possible, inhaling only when the first feeling of discomfort starts. He should then rest and breathe normally for several seconds. Next, he should breathe in as deeply as possible. The air is to be kept in the lungs until it is uncomfortable to do so. This alternation completes one cycle of breathing. It may be repeated a number of times, if necessary, but never for a longer period than ten minutes.
-- Notebooks Category 5: The Body > Chapter 6: Breathing Exercises > # 3
-- Perspectives > Chapter 5: The Body > # 25