A discovered attack happens when moving one piece reveals an attack from another piece that was hidden behind it. The moving piece is free to create its own threat, so the defender faces two attacks at once. When the revealed attack is a check, it's a discovered check — and if the moving piece also gives check, a double check.
The discovered attack is the closest thing chess has to a trapdoor. One piece steps aside, and an attack that was standing behind it the whole time suddenly exists. What makes it vicious is that the piece that moved doesn't have to move quietly — it can make its own threat, and now there are two.
White's bishop on b2 points down the long diagonal at Black's queen on g7 — but White's own knight on e5 stands in the way, so nothing is happening. Then the knight jumps to d7, attacking the rook on f8. Two things are now true at once: the knight attacks the rook, and the bishop, unmasked, attacks the queen. Black saves the queen; the knight takes the rook.
The mechanism is the same double attack logic as a fork, with one upgrade: the two threats come from two different pieces, so no single defensive move is likely to parry both.
When the piece behind attacks the enemy king, the discovery becomes a discovered check — and the moving piece becomes temporarily invincible. It can grab anything on its path, because the opponent must answer the check first. A rampaging piece under repeated discovered check (the 'windmill') can strip a position bare.
Rarer and stronger still is the double check, where the moving piece gives check too. Against a double check there is exactly one kind of legal reply: the king must move. No blocking, no capturing the checker — some of the most famous mating combinations in chess history, including the classic smothered mate, run on this.
The warning sign is a battery in waiting: an enemy bishop, rook, or queen on the same line as something valuable of yours, with only one enemy piece between them. That piece is a loaded spring — every square it can jump to is a potential discovery. Train the same line-scan you use against pins and skewers: who is aligned with what, and through whom.
Discovered attacks you never saw coming leave a distinctive mark in your games — a sudden evaluation cliff on an innocent-looking move. Chessdock finds those cliffs in your play and turns them into puzzles, so the loaded spring becomes something you spot a move early.