As the only way to parray Rf7+, Ke8 - Qxg8 # ( it's explained in solution)

After Qe8, if Rf7+, then Qxf7; exf7, and finally Rxh8 ( so the game would be equal) 

That's the best defence for black.

But white continues with an unexpected shot Rxd6. (solution explains all the moves..)