Or that calm is retained by the controlled exhalation or retention of the breath.
The mind is also calmed by regulating the breath, particularly attending to exhalation and the natural stilling of breath that comes from such practice.
Patanjali here is talking about pranayama, or the control of the movement of prana (the vital life-force) through our breathing. Be mindful of the breath, and you will regulate your mind. We have all experienced this when feeling worried or anxious, if we purposefully take a few deep breaths then it can help to calm us.
There is some debate as to whether Patanjali meant simply observing the breath or whether he actually meant retention of the breath, kumbhaka. Swami Satchidananda believes he just meant regulating the breath, rather than detailing any specific breathing exercise. Being mindful of the breath going in and out can bring serenity of mind.
This excellent blog, which I have referred to many times whilst reading the sutras, says that:
Pranayama is often translated as breath control. The root ayama actually means lengthening. Thus, pranayama more specifically means lengthening the life force.
This is in contrast to Zen Buddhism, which does not stress the end result of meditation. There is no goal in Zen, we meditate to see things as they really are. Serenity of mind may be a pleasant by-product of this, but should not be the ‘goal’ (there is no goal). Some days it may bring serenity, some days it may not. Both are ok. There is no expectation in Zen that there will be any particular result or side-effect. It is what it is, and that’s ok.