When there is calm, there arises for this ascetic the ending, the destruction, of all pains, those pertaining to the self and the rest. Further, of one whose mind is calm, whose inner instrument is at ease, the understanding soon stands wholly firm: like space, it stands firm on every side, becoming unmoving in the very nature of the Self. Since one of calm mind, of settled understanding, has done what is to be done, one should, with senses freed of passion and aversion, rightly engage with the unavoidable objects not forbidden by scripture. This is the sense of the passage. This calm He praises.

When this man's mind has serenity, there comes the destruction of all the pains that contact with matter brings. For one whose mind is serene, whose mind is free of the faults that obstruct the beholding of the self, at that very moment the understanding that bears on the self set apart stands firm in Me. So when the mind is serene, there is destruction of all pains.

This is a brief sub-gloss. For a fuller reading of this verse, see Madhusūdana, Śaṅkara, or Rāmānuja above.

How does the destruction of all pain come about through mere serenity? Because the buddhi of one whose mind is serene comes to rest, that is, it makes a right standing through the direct vision of Brahman. 'Serenity' is, for the most part, a turning away from objects of itself.