What checks can Black give with his rooks? None. What loose pieces does White have? Two: the bishop and knight (as usual, we’re setting pawns to one side), and they are lying on the same rank—the second, perfect for a double attack by the rook. As this position and the previous one both show, rook (or queen) forks on the opponent’s second rank are a fairly common pattern because pieces there often are loose: they frequently get no protection from their fellow rooks, which are on the first rank, and they can’t be protected by pawns. Naturally this pattern lends itself to double attacks by the queen as well; the rook’s moves, like the bishop’s moves, are a subset of the moves a queen can make. In any event, Rd2 wins Black a piece.