By now the idea here should be easy to spot: Black’s rook and c7 knight are a square apart on the same rank, and White’s pawn on d5 is a square away from being able to fork them. The problem is that d6 is occupied by an enemy pawn. How do we get rid of it? In familiar fashion: take something it protects, forcing it to move to recapture. So White plays NxN; Black recaptures d6xN; and now the fork d5-d6 wins Black’s other knight.