What checks can White’s rook give? Two: Rxf7 and Rd8. The interesting question about Rd8 is whether a Black piece can be moved elsewhere onto the back rank and thus turn that check into a double attack. White only has one other piece to work with—the bishop; so try attacking something with it. Bb5 threatens the Black knight and forces it to move. Where can it go? Its only safe square is b8—and once it’s there, Rd8 forks and wins it.
At several points in our studies we will see this principle used: an attack on an enemy knight that forces it to move somewhere useful to you. The thing to remember about knights is that their range of motion is inherently limited; they never can make it to more than eight squares, and where (as here) they are near the edge of the board that number gets even smaller. This often makes it easy to figure out where a knight will have to go if attacked, and thus turns attacks on enemy knights into very useful forcing moves.