Sutra 2.51: bahya abhyantara vishaya akshepi chaturthah
- bahya = external
- abhyantara = internal
- vishaya = reach, faculty, realm
- akshepi = surpassing
- chaturthah = the fourth
Translated loosely into English;
The fourth (pranayama) surpasses beyond all realms, both internal and external.
When all external and internal realms are surpassed, it is called the 4th pranayama.
Patanjali refers to pure prana, which is beyond inhalation, exhalation and the space between, transcending breath to the most subtle life force. The student is absorbed by prana itself. The breath continues but prana has stopped and the subtle intestine movement, peristalsis, is arrested. (The intention behind fasting)
An excellent example of this by Swami Bharati (found at http://www.swamij.com)