Play has gone 1. QxR, KxQ; 2. Bc5+, Qd6; and then 3. RxQ. This last move isn't a check, but with careful play it still gives White a forced mate: 3. …Ke7 (the king tries to escape from the threatened Rd8#); 4. Rh1-d1 (bringing in reinforcements and threatening mate next move with the discovered check Rf6), Nd5 (interposing to block the rook now on d1, and thus preserve e7 as a flight square); 5. Rd7+—and now Black is about to get mated, though in ways that may be visually unexpected. If he moves his king to e6, for example, White mates with c4xN: a pawn administers the fatal blow. If Black instead moves his king to e8, White plays Rb7, discovering check by the bishop on b5; then if Black interposes with Bd7, White mates with RxRb8. Or Black can move his king to d8, in which case White mates most efficiently with Be7. Yes, it gets intricate. But in the initial position you might well go forward simply upon observing the two rooks won by 3. BxQ.