Unlike the fork, discoveries need two attacking pieces. In a discovery, you move one piece (unmasking piece) out of the way of another (unmasked piece), unleashing attacks on two enemy pieces at the same time. The enemy can only protect one, you take the other.
As you can see, it takes more work to set up this kind of attack, but it’s therefore also easier for your opponent to miss.
How do I do it?
Discoveries can be done by bishops, rooks, knights and pawns. Queens can also be involved, but only in check discoveries.
The most common ones are with bishops, either as unmasking or unmasked piece. Bishops have a very wide range of attack, which is usually only limited by your own pawns or pieces. That’s why moving one of these out of the way creates all sorts of possibilities for a bishop.
For the non-diagonal attacks, rooks are the way to go. They are usually pointed at the opponent’s king or queen from the back rank, with only one of your knights/bishops separating them. Move that out of the way, and you can launch a discovery.
Knights and pawns can only be used as unmasking pieces. Knights can jump over pieces, and pawns have only a one-square attacking range – nothing changes their attacking range. Nevertheless, moving a pawn forward to launch a discovery is usually a better plan than using two pieces in an attack. That’s because you want your pawns to move forward anyway, and they are worth less than any piece (in case things go wrong).
Just as with forks, discoveries where one of the threatened enemy pieces is the king, are the best. The king has to move, which means you are sure you can capture the other piece you were attacking.
Check discoveries are the only moment you can (and should try to) use your queen. The queen has the most wide attacking range of them all, which means she makes it easy to set up a discovery. Usually a knight in front of your queen is moved to check the enemy king.