Lesson
White to move. Main line: 1. Nf7#
Nf7 is mate because every escape square is taken by Black’s own army: the rook holds g8, the pawns hold g7 and h7. The knight is the one piece whose attack cannot be blocked.
This is the final picture of the famous smothered mate. The full version — where a queen sacrifice forces the rook to g8 first — is in the mate sequences section.
The knight’s immunity to blocking is what makes it the only piece that can finish this position. Against any other attacker the units on g7, h7 and g8 would be shelter; against a knight they are the walls of the cell.