“…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.