The discovered attack

A discovered attack is two threats from a single move. One of your pieces is blocking the line of another; when the front piece moves away, the piece behind it suddenly attacks down the open line. The killer detail: the piece that MOVES can make its own threat at the same time, so the opponent can't deal with both.

In the diagram, the bishop on b2 aims down the long diagonal at Black's queen on g7, but the knight on e5 blocks it. Move that knight with a threat of its own, and the bishop's attack on the queen is 'discovered' — Black can't save the queen and answer the knight too. A discovered CHECK is the deadliest version of all.