What can White attack with his bishop? Be4 attacks a knight, which might make a good start for a double attack if the knight were loose; but it is protected by the rook at d8. Nor would the bishop then be attacking anything else. Still, the sight of the bishop attacking an enemy piece while also aimed at h7 should stimulate your imagination. If a loose piece could be substituted for the knight, and if the king could be pushed onto h7 as it was in some earlier problems, White would have a fork. So experiment a little with those possibilities.
When a target is protected, one way to loosen it is to take it with another piece and allow for a recapture; here, 1. NxN, RxN and White has half of a bishop fork in place. Next White turns to any checks that would force the king onto h7 and finds Ra8 (remember that the Black rook no longer would be on d8). After 2. Ra8+, Kh7, White plays 3. Be4+ and wins the rook.
This is a position we already saw early in the chapter, where it was advanced to its last step; the point of repeating it here is to enable you to see what the same fork looks like a couple of moves away. Notice, too, that the sequence succeeds here only in the move order described; trying to move the king first, and then performing the exchange of knight for rook, would not have worked. So remember to look for your checks before and after you imagine captures you can make.