प्रशान्तमनसं ह्येनं योगिनं सुखमुत्तमम्।उपैति शान्तरजसं ब्रह्मभूतमकल्मषम्॥
praśānta-manasaṃ hyenaṃ yoginaṃ sukham uttamamupaiti śānta-rajasaṃ brahma-bhūtam akalmaṣam
Word by word
The verse says
For this practitioner, his mind at peace and his restlessness quieted, stainless, having become the infinite — the highest happiness comes to him.
In plain words
The result of the bringing-back.
Śānta-rajasam — whose rajas has been quieted. The strand of restlessness and drive, named back in Chapter 3.
And notice the verb: the happiness comes to him. He does not go and get it.
Where you've felt this · in relationships
The argument you used to rehearse in the shower has stopped arriving. You did not win it and nothing was settled with anybody. It simply stopped being scheduled.
Going deeper
The direction of the verb is the quiet surprise in this verse.
Upaiti — it comes to him, it approaches. Every practical instruction in this chapter has been something the person does, and then the result is described as arriving on its own.
That is consistent with the lamp. You shut the door; you do not steady the flame. The steadiness is what happens when you stop doing the thing that was preventing it.
Śānta-rajasam ties it back to Chapter 3's account of desire, where rajas was the strand that makes a person restless and driven. It is not being destroyed here. It is being quieted, which is what you do to something that stays.
And for a reader the practical form of this is modest. You are not required to produce peace. You are required to stop supplying the draught.