Think back to Morphy’s mate. It involved pressure against h8 from a bishop coupled with pressure down an open g-file from a heavy piece. The role of the heavy piece in that sequence also can be filled by a knight on h6; thus Black is mated in the diagram by White’s knight just as he would be by a White rook on g3.
Since our goal is not only to see the mating patterns in final form but also to understand the moves that lead to them, reflect for a moment on how the knight could have gotten to h6. It had to come from on f5 or g4—which is one reason why those are considered strong squares for a knight.