Nice double-check rook sacrifice. 1.Rd7+ limits black to only one move: 1...Kxd7. White has to be careful of black's mate threat. White must corral the black king not against the back wall, but towards the knight (and away from the black queen). Ultimately, d6 is the choke point, where the white queen can deliver mate when the black king reaches the c6 or c7 square.