In the last two positions the discovered attack required a preliminary exchange that both improved the target the unmasked piece would attack and opened a line for the unmasking piece to use to make a threat against the enemy king. We now look more closely at this last principle: identifying lines that need to be opened to make a discovery work, and clearing them with exchanges and threats.
In the position to the left, begin with the standard identification of the kernel of the discovery. Here it's on the d-file, where White's rook is aimed at Black’s queen at the other end of the board. The difficulty—both in seeing the kernel and in perfecting it—is that White has two bishops in the way. If one of the bishops could be cleared from the file in a manner that is time-consuming for Black, the board might be set up for an effective discovery by the bishop that remains. Each bishop has a possible threat (try imagining the board with one of them removed, then the other): the d3 bishop can go to c4 and threaten Black’s rook on a2, and the d4 bishop can capture Black’s knight on f6 and threaten to take another piece from there. So first White plays Bc4, requiring Black to respond by moving the rook to a8 or perhaps a5; and now the way is clear for White's other bishop to play BxN. Black has to spend his reply move saving his queen (this is one of those discoveries where the threat by the unmasked piece is greater than the threat by the unmasking piece).
Notice that after White plays BxN, Black could try to take the offensive by playing his e7 bishop to c5—a move that seems to expose his queen to capture, but actually is quite safe because it gives check (when White played BxN, he created an open diagonal to his own king). Now if White moves his king, Black can play QxBf6; in other words, the Black bishop's move to c5 was another discovered attack. But White has an answer: he can block the check and save his piece by simply retreating his bishop to d4, having already won a piece. We see again a recurring point: don’t forget to look for checks your opponent might be able to throw into the middle of the sequence you are planning. They can ruin your plans by buying him time to move his pieces out of harm’s way. It’s not a problem here, however, because White has an excellent answer.
Now think about one other way things could go. Black could respond to the initial move 1. Bc4 by moving his queen to a8 to protect his rook; his plan this way would be to lose just the exchange rather than a whole piece. But play it through in your mind’s eye: White plays 2. BxR; Black replies with QxB; but now the queen ends up in cramped territory, which should worry Black. What happens if White throws an attacker at the queen with Ra1? The queen turns out to have nowhere safe to go. So Qa8 as a first response for Black doesn’t work out. Again, though, the important lesson is general: if you attack an enemy piece as part of a double threat, realize that he might be able to reply by adding to its protection as well as by moving it. If he does add to its protection, imagine going ahead with the capture and permitting him to recapture, and ask how the board would then look. Obviously this practice would be just as important if you were playing the Black pieces here; else you might lose your queen.
Finally, let’s return to the initial diagram to emphasize the most basic point of the position: if you see the skeleton of a discovered attack but with extra pieces in the way, think about how you might clear the obstructions in a forcing manner that cuts down your opponent’s choice of replies—i.e., with checks, captures, and threats. Here a simple one-move threat by the d3 bishop forced Black’s reply and made possible a classic discovered attack. More broadly the position shows the importance of seeing the kernel of a discovery even when there are other pieces also cluttering the line.