Where does Black have the makings of a discovered attack? There is nothing on the files or ranks, but along the g1-a7 diagonal Black has a bishop masked by a rook. Does the bishop have a suitable target if the rook moves? Yes, in White’s unprotected knight. As discussed in more detail in our work on forks, an unprotected or “loose” piece always calls for a look at whether it can be taken with a tactical maneuver. Here the question is whether Black can vacate his rook from c5 in a manner that is time-consuming enough to allow him to take the knight a move later—and that doesn’t lose the rook. (Since the payoff of the discovery is just going to be a knight, Black can’t afford to sacrifice his rook the way he did in the previous positions.) So Black looks for safe rook checks and finds Rc1+, where the rook is covered by Black’s bishop at e3. The move forces the king to move to a2 and then allows Black to play BxN safely.