Discovered attack
Move the front piece and the piece behind it does the work. Strongest when the front piece also makes a threat.
White to play
Nb5+
The knight steps aside and the bishop behind it gives check. Black must answer the check, and then the rook on c7 is taken.
What to look for
Look for your own pieces standing on a line between one of your long-range pieces and something of theirs worth hitting. The front piece is the one to move.
When it applies
When the discovered line gives check, because then the opponent has no time to deal with whatever the front piece went on to attack. That combination wins material almost every time.
How it goes wrong
Getting the order wrong. If you move the front piece somewhere harmless, they answer the discovery and you have gained nothing but a tempo loss.
Where it came up
Not cited in your current game.
Practise it
No positions yet. When you miss this idea in a game, the position it cost you shows up here — and in the practise set — with the answer hidden.