What checks can White give with his queen? Diagonal moves won't work because the queen is on a dark square and the king is on a light one. But White does have a check—his only one on the board—in Qxf5. So now you ask whether the move also would attack anything else and notice that the queen would be pointed at the bishop on d7. The bishop isn't loose, but it's attacked once and protected once. Thus consider a preliminary exchange that might leave behind a loose piece: if White plays 1. RxB, RxR, the result is a loose Black rook on d7. Now Qxf5+ forks the rook and nets a piece and a pawn.