We have seen how to deal with obstacles that may stand in the way of a normal discovered attack—how to create good targets for each piece, and how to open blocked lines. The same methods come in handy when building discovered checks, so there is no need to rehearse them here in detail; but since their use here sometimes looks a little different, a few examples may be useful. First, discovered checks often have to be created by making a threat or sacrifice that brings the king within range of the stationary piece. And since an unmasking bishop can go after enemy pieces that sit on the same color squares where it travels, you sometimes have to force a target onto one of those squares.
In the frame to the left, Black has the kernel of a discovered attack on the g-file: the rook behind the bishop. If Black unmasks the rook it will attack the pawn at g2, adjacent to White's king; and if the king itself were on g2, would of course have a far more potent possibility: a discovered check. How to move the king? Look for other pieces Black can use to check it, capture pieces next to it, or both. There is one option: Qxg2+. White’s only legal reply is KxQ. Now that the king has been lured onto the g-file, all that remains is to find a good target for the unmasking piece—the bishop on g7. It travels on dark squares. White’s queen is on a dark square. So Bxe5+ takes a bishop, puts White’s king in check, and wins back White’s queen after Black’s next move.
By the way, if you have studied the chapter on bishop forks, another idea for Black should cry out for attention here as well: BxB+, forking White's king and queen. What makes it especially interesting is that if White takes the bishop, Black then appears to have mate: Qxg2#, with cover provided by the rook on g8. Indeed, Black seems ready to mate soon no matter how White replies to BxB. But not quite; for the surprising actual result of Black's bishop fork is that it permits White to mate! The point to notice is that when White replies to the fork with QxB on e5, he checks Black's king and seizes the initiative. Black has to play Rg7 to block the check. Then White plays Rg3; this not only blocks Black's mate threat against g2, but allows White to mate a moment later on g7 with QxR. There is nothing Black can do to stop it; his queen is too far out of position to help. The moral: you must be especially careful to observe whether any of your opponent's replies to your ideas will put you in check. It can ruin everything.