Figure[Black to move]

Spotting the idea for Black here should be easy. You just have to recognize that the knight on d6 is not so much “in the way” of the d8 rook as masking it and creating the kernel of a discovery. White’s back rank is weak, as d1 has no defenders; the queen on f2 looks ready to help, but then you see that it’s pinned. So if the d6 knight moves, Black threatens RxR#. This means that Black has something close to a discovered check and that his knight has a more or less free move. He plays Ne4 and then takes the queen after White fends off the mate threat. Notice that this “fending off” has to be done carefully:

(a) QxQ is not one of White’s options; RxR then mates for Black, since his knight now seals off f2 as a flight square for White’s king.

(b) If White moves his h2 pawn to h3 to create a flight square, Black has QxQ+ followed by Qg3+ and then RxR#, with the queen sealing the White king onto the back rank.

(c) If White plays RxR, Black has QxQ+, then Qe1# after White’s king moves to h1.

(d) What White can do in reply to Ne4 is move his queen to d4. Black does not then capture it; since it is pinned he attacks it with a pawn, e6-e5. Then White can play f3xN, and Black answers e5xQ—winning a queen for a knight.