Lesson
White to move. Main line: 1. Nxd4+ Kf8
The knight on e2 masks the rook on e1. So 1.Nxd4+ captures the rook and simultaneously discovers check along the e-file — and because the check comes from the rook, not the knight, Black must move the king and cannot recapture the knight on d4.
A discovered check is the strongest kind of discovery: the moving piece is immune, because the opponent has to answer the check first. Grab the most valuable thing you can reach as you unveil it.