By Dan Gartland The NBA playoffs essentially begin on Tuesday. Good morning, I’m Dan Gartland. I finally understand how the NBA play-in tournament works. If you’re reading this on SI.com, you can sign up to get this free newsletter in your inbox each weekday at SI.com/newsletters. The NBA playoffs essentially begin tonightThe NBA play-in tournament is back for a second year—and it begins tonight. While I think it’s undeniable that the addition of the play-in—by expanding the postseason field to 10 teams—has improved the quality of play at the end of the regular season, it’s still a new concept…