I don't know all the specifics. I do know he has elite speed, seems to come through in the clutch, is loved by his teammates (Jose Ramirez recently says he wants him to stay with the team so badly he'd "be willing to foot the bill") and played very well down the stretch.
I do want to add to this, Kwan takes a lot of pitches, our third batter is Jose, so you know they are nibbling when throwing to him and he can take some pitches for this reason. So Rosario being more of a free swinger "should" get better pitches to hit.
