I hear this term so much but I don't really understand it. Taken at face value it means they believe in the system now but they didn't before. What does belief have to do with football (or basketball) success?
Is it rather just a charged way of saying that they have mastered the playbook while the people who departed (or had a bad season last year) didn't? To me, any outright failures from last year should be due to the team's unfamiliarity with the plays and schemes. As in, if there were an exam on the gameplan they would get a C (or a D, or an F). This makes more sense to me than saying it was caused by a lack of faith.
I'd chalk up the term "buying in" to journalistic rhetoric, but players use the term all the time. Does anyone know what "buying in" to a system actually entails?