When the pawns are blocked and the defender's pawn sits on the colour of their own bishop, the knight attacks what the bishop cannot defend from a distance and the king invades on the other colour.
The chapter on bishops gave the bishop its due with pawns on both wings; this drill restates the comparison from the knight's side. Black's bishop on a1 is dark-squared and Black's only pawn, c5, is on a dark square. White's pawns are c4 (light, untouchable by the bishop) and f4 (dark, but easily defended). The structure is blocked, so the bishop's long range is wasted.