Lesson
White to move. Main line: 1. Nf7+ Kg7 2. Nxd8
From f7 the knight attacks both h8 and d8. The check forces the king to move — it cannot capture the knight and nothing can block a knight check — and the queen on d8 falls next move.
The pattern to memorize is the geometry: a knight on f7 (or f2, c7, c2 for the other corners) forks a castled king and a queen on its original square. In real games this often appears after a sacrifice deflects a defender away from f7.
Presented as a Puzzle replay, the thinking pause gives viewers a moment to find Nf7+ themselves before the reveal.