Yoga Sutra 1.34 pracchardana vidharanabhyam va pranasya

“…by maintaining the pensive state felt at the time of soft and steady exhalation and during passive retention after exhalation.”

The sutra refers to a method of breath control which also relaxes the mind allowing for control of the mind, which keeps it from fluctuating. I have experienced this serene state while practicing pranayama while retaining the exhale, or emptying the lungs before an inhale. I feel like I am releasing all of my thoughts with the exhale and during the exhale retention I am completely empty creating a serene, relaxed state. It is an amazing feeling of being centered without any fluctuation. The only awareness I have is of my heartbeat and the blood and prana running through my body. During this time I feel part of something much bigger than just my body. I feel connected to life.

Afterward the calmness that I experienced during pranayama stays with me, but how long depends on my ability to allow external interactions or internal fluctuations to overtake the calm.