Figure[Black to move]

Play from this point goes 3. …Bd5 (useless interposition); 4. BxB+ (not QxB+—d5 is guarded by Black’s rook), e7-e6 (useless interposition); 5. Bxe6+, Rf7 (useless interposition—though it gives the king a flight square); 6. BxR+, Kf8 (forced); 7. Nh7+ (don’t forget the knight), Ke7 (forced); 8. Qe6#. (If Black instead plays 4. …RxB, which might have occurred to you, this merely gives White a safe chance to play 5. QxR+ and mate even more quickly: 5. …e7-e6 (useless interposition); 6. Qxe6+, Rf7 (forced useless interposition); 7. QxR#. It is worth spending the time needed to visualize these mating sequences from the original diagrammed position just for the sake of getting some exercise. But if Black is alert he of course will avoid all this, replying to BxN with Ba6 and thus counterattacking against White’s queen. This way he loses “only” a piece.

Conclusion: after 1. Ra1, Nd4 (Black’s best reply), White should play the (a) line above, not the (b), because (a) wins him a pawn as well as a piece.