The Discovered Attack
A discovered attack is created when one piece moves out of the way to reveal an attack from a piece behind it. Because the moving piece can make its own threat at the same time, two things are attacked at once.
The knight on d5 can move with tempo while the rook on d1 attacks the rook on d8 behind it.
Why it is so strong
The opponent must deal with the revealed attack, but the moving piece is free to capture or threaten something else entirely. A discovered check is especially powerful: the opponent must respond to the check, letting the moving piece grab material.
Double check
The strongest form is the double check, where both the moving piece and the revealed piece give check at once. The king must move — it cannot block or capture both — which often leads to forced mate.
Practice this pattern
Real positions where a the discovered attack wins — tap a puzzle to solve it, or train the theme endlessly.