Embryonic breathing is sometimes called "stopping the breath". But it is important to differentiate that "stopping the breath" actually does not mean the same thing as "holding the breath". There are several Daoist practices that involve holding the breath to increase lung capacity, but Embryonic breathing is actually a bit different. Embryonic breathing means that your breath feels like an effortless, internal movement. Your breathing becomes so natural you are not aware that you are the one breathing, like a new-formed child in the womb. When I practice this type of breathing, I find that my spirit and breathing become aligned, I become less aware of myself sitting there. Click to learn how Chinese medicine treats Sweating Syndrome.
I become present in the moment, and am only aware of the breath. This is a very useful type of breathing for meditation. Focusing the mind too sharply on your breathing can prevent the naturalness of breath needed for Embryonic breathing, so instead of diligently practicing this technique, I find it more useful to let it happen naturally. If you use one hand to hold a feather under your nose while practicing this type of breathing, it will not move, your breathing is so calm and subtle. Embryonic breathing is also called nei hu xi (meaning 'internal breathing').