In Daoist practices, there is an enormous amount of breathing techniques intended to treat diseases, accumulate and control the movement of Qi. Exercises may differ from each other in very different parameters: the type of breathing is used (forward, inverse, breast, skin / corporal, sighing, mixed breathing); rhythm, the presence of breath holding or lack of it, and when it is done (after the inhalation, exhalation or both); methods of internal work during breathing exercises; presence or absence of the dynamic aspect of pronouncing, and special sounds. One of the most difficult breathing techniques is the “embryonic breathing”, the essence of which is to ensure that the practitioner ceases to breathe with his lungs, while his heart starts to beat very much slower, and may eventually stop completely. This type of breathing is needed for high levels of practice, when you want to stay in a deep state of meditation for many days or weeks.
You can read Qigong Meditation Embryonic Breathing here