General concepts of Pranayama

As I learned from my teacher, a student of Yoga passes on to Pranayama after mastering Asana. Asana constitutes the third part of the course of Yogic study, which is divided into eight parts (Ashtanga Yoga).

Pranayama, the fourth aspect yoga, means a pause in the moment of the breath.  In Sanskrit Prana means breath and Ayama means pause.  I lot of modern Yoga books translated Pranayama as a subtle psychic force or a subtle cosmic energy.  Some of the Hatha texts, very often use the word Prana to indicate a subtle life force. But this they do when they talk about the force awakened  by the process of Pranayama and not of Pranayama itself. In the yogic literature Pranayma means a pause in the movement of breath. 

Patanjali's Four Types of Pranayama

Patanjali notices four types of Pranayama. The distinctions is based on the nature of the breathing pause.

When the pause is made after  a deep exhalation, this constitutes the first type of Pranayama  . The second type of Pranayama is when the pause comes after a deep inhalation. In both the cases the Yogic student has to make an effort to hold his breath either in or out. But the third and the fourth types of Pranayama the student is not required to make any special effort for holding his breath.  The respiratory movement should stop all at once, when the student wants it to stop, the pause should be performed without any physical effort on the part of the student. The difference between the third  and the fourth types is that in the third type come all at once, while in the fourth type of pranayama  the pause should start after many inhalations and exhalations preceding it. The absence of effort in maintaining the pause, should happen in the third and in fourth types of Pranayama.
In the language of the later Yogic literature, the first type is called Bahya Kumbakha, the second type, Abyantara Kumbhaka, and the third and fourth types are called Kevala Kumbhakas.

 Swami Kuvalayananda