1. Nf3

(The Knight moved from d4 to f3.) This was necessary because prior to this move, 1. Ra8 would have been met by Bxa8, and 1. Rh8 would have been met by Rf8. White now threatens to make either move and give mate because the Knight blocks both Black's Rook and Bishop. Black must attempt to eliminate this block, but can only eliminate it for one of the two pieces:

1. ... Rxf3 leaves the Bishop blocked (by Black's own Rook), so White replies 2. Ra8 mate.

1. ... Bxf3 leaves Black's Rook blocked (by Black's own Bishop), so White replies 2. Rh8 mate.

Any other move by Black leaves White with the choice of 2. Ra8 mate or 2. Rh8 mate.

This tactic is known as interference.