Lesson
White to move. Main line: 1. f8=N+ Kh6 2. Nxd7
The promotion piece is a choice, not a formality. On f8 a knight attacks h7 and d7, so f8=N+ creates a royal-style fork immediately. A queen on f8 would check, but it would not attack the queen on d7 with the same tempo.
Black moves the king out of check with ...Kh6, after which Nxd7 collects the queen. White converts a pawn into a knight and wins nine points of material, a much greater return than choosing the nominally strongest promotion piece.
Before every promotion, compare all four pieces for checks, forks and stalemate effects. Underpromotion is rare because a queen is usually best, but its rarity is exactly why the knight’s unique movement is easy to overlook.