When a piece moves out of the line of a rook, bishop or queen behind it, the hidden piece attacks. If the moving piece also gives check, it is a double check, which only a king move can answer.
Key principles
- The front piece can go anywhere, ideally with its own threat
- A double check can only be met by moving the king
- Batteries (two pieces on a line) set up discoveries
Common mistakes
- Moving the front piece to a square where it is simply taken
- Overlooking a discovery against your own king