Lesson
White to move. Main line: 1. Nc4+ Kg8 2. Nxd6
Nc4 unmasks the bishop on b2 — check — while the knight lands on a square attacking the queen on d6. Black must deal with the check first, and the queen is taken for free.
A discovered attack lets two pieces strike in one move; when the unmasked piece gives check, the moving piece can do anything it wants. Look for your own pieces standing on a teammate’s line: each one is a loaded spring.
The strongest version is double check, where the moving piece gives check as well. Then no capture and no block can help: the king must move, and the discovering piece is free to land wherever it likes, however undefended the square.
If Kh7: Nxd6