In broad terms, the mind slowly gets remold in a very beneficial way. It's a bit like standing in a huge, noisy crowd where you couldn't make sense of much things because everything's moving, but with such training you slowly gain some distance, like you step back and you see the crowd from slightly above/afar (while still being present) and can notice the general movement as well as (increasingly with time) details, which somehow builds a feedback loop that makes you grow calmer. Everything then trickles down from that.