Black’s rook is unprotected and so makes a fine target. Now you look for queen checks available to White and find many on this open board: Qd7, Qb7, Qb4, Qg5, Qc5, Qe8, Qe5, and Qe2. But we are only interested in checks that also attack the Black rook, and from a square that the rook cannot attack. Qe2+ works—a study in the importance of considering backward moves as well as the more obvious attacking moves that push your pieces toward your opponent’s end of the board.