Mastering the discovered attack partly is a matter of getting to know each of your pieces again—as maskers of attacks by other pieces. Thus you may at first think of the bishop as a piece that attacks along diagonals and create double attacks of the sort we saw in the chapter on bishop forks. That’s accurate as far as it goes, but another part of the bishop's power comes from its ability to unmask attacks along files and ranks by rooks and queens. When you look at a bishop you want to see more or less automatically not only where it is aimed but also whether any heavy pieces lie on the rank or file where it resides. You likewise want your eyes trained to notice when a bishop or queen lies on the same diagonal as one of your rooks; and when a rook, bishop, or queen lies on the same line with one of your knights; and when one of your pawns conceals a bishop on its diagonal or a rook or queen on its file. You get the idea: sensitize yourself in general to alignments of your pieces and your opponent’s pieces—both on the board in front of you and on the board as it would look after one or several moves in a series you imagine. Noticing alignments is the key to seeing chances for discovered attacks. It also is the key to seeing chances for pins and skewers, as we will learn in more detail in the next section.
The execution of a discovery is a climactic moment during a game; on most of your moves you will have no such opportunity. So what do you do then when you have no way to win material with tactical strikes? You play strategically, making positional moves that increase the strength of your pieces and make eventual tactical shots more likely. From our studies of discovered attacks we can infer some lessons about sound strategic play: the significance of open lines; the usefulness of centralizing your pieces; and the importance of a king’s vulnerabilities. Each of these considerations can be seen from an offensive or defensive standpoint, but for present purposes we mostly will look at them from an offensive point of view.
Discovered checks by bishops require, first, open lines for the rooks and queens that the bishops unmask—in other words, lines unobstructed by pawns. Think first about open files. Rooks depend on them; one thing you learn early in chess is that as powerful as rooks seem to be, they usually do little good so long as they sit behind pawns (unless the pawn is on its way to promotion, but set that possibility to one side for now). Moving a rook to an open file thus vastly increases its strength. Another thing you soon notice is that getting two rooks onto the same open line multiplies their powers further. The point of our studies here is that discovered attacks are still another way to play on open files. You can put a bishop or knight there in front of your rook or queen, or in front of the squares where your rooks or queen may go; in this way you create potential energy on the file, as you have the makings of a double attack of the discovered variety if targets can be brought into view at both ends.
The same goes for diagonals. You can play on a diagonal not only by putting a bishop there or a bishop plus a queen, but also by putting a rook or knight in front of those pieces. The rook or knight can't travel on the diagonal, of course, but can jump from it and thus create two attacks at once: the tactician’s dream.
Once you appreciate the great uses of open lines, the next question is how to create them. It’s all about pawns. If there are no open files and diagonals, your job is to create them through pawn warfare. Often a pawn move or exchange will be most significant because it will open a file halfway (eliminating one of the pawns on it) or completely (eliminating both pawns), leaving it open for occupation and domination by a rook. Pawns also determine whether diagonals are available for your bishops, whether to create forks or pins or—here—discoveries. This is especially true when either side has a cluster or “phalanx” of pawns, as pawns so arranged will tend to sit on squares of the same color; that is how they protect each other. Their arrangement will determine whether the bishops that travel on those squares will be active or inert. The lesson is to think about the implications of pawn moves and exchanges for the mobility of your rooks and bishops—and those of your opponent.
So getting your pieces onto open lines is one way to create a fertile environment for discoveries. Another element is centralization —establishing pieces near the middle of the board. A discovery requires an arrangement of three pieces: an attacker, a target, and a piece of yours that lies between them and that can leap out of the way. We have seen lots of ways those pieces can be arranged; they all can be clustered at one end of the board, or two can be at one end and one at the other. But in general a knight is most likely to be in position to unmask an attack by another piece when it is more or less centralized, for when it is near the middle of the board it has the best chance of ending up between a friendly and an enemy piece. The same principle holds for most of the other pieces when acting as unmaskers. Rooks need lines unblocked by pawns before they can be unmasked and threaten anything. But if a rook is going to unmask an attack by a bishop, it needs more than an open line; it needs to get out onto the board where it can get in front of a bishop or queen.
Finally there is the matter of targets. We have seen many times that a good place to start looking for tactical opportunities is the enemy king's position: its exposure, ways that it can be forced to move, or that lines toward it can be opened. The point of the investigations is not necessarily the thought of checkmate, which may be nowhere in sight. The point is that double attacks of all kinds—including discoveries—work best when the enemy king is at one end of them because your opponent’s choice of replies is then so limited.
The point has several implications for strategic play. One is the value defensively of castling early, and the advantage to be had if you can prevent castling by the other side. Disrupting the pawn cover in front of the enemy king likewise can have great value, as the king then becomes more exposed as a target at one end of discoveries you may arrange (as well as forks and pins). There are lots of ways to ruin a king's pawn cover; some of them involve sacrifices, usually as a run-up to a tactical blow you have planned. But on occasion you may be able to tear the cover apart without a sacrifice, and without any particular follow-up in mind, just by forcing exchanges of pieces that are protected by the pawns in front of his king, or by sending your own pawns forward to harass those of the enemy.
Another related point is the value of coordinating your pieces to create pressure on the enemy king’s position. This is not always the best strategy; sometimes the king’s position is secure and your advantages lie elsewhere on the board. You make plans in the sectors where you have the best promise of success. But offensive pressure against the enemy king can create prospects for all kinds of tactics that involve busying your opponent with threats in one place while you also create simultaneous threats elsewhere. Forks are one example; discoveries are another. And having pieces coordinated—i.e., aimed at the same area of the board, or otherwise supporting each other's work—tends to be a lot more valuable than having them fire in different directions, in part because their powers can then be combined to make those tactical shots work. Maybe one piece makes a sacrifice to set up a target for another; maybe two of them combine to create a mating threat while also making a threat elsewhere; maybe one can mask and unmask the other.