What do discoveries look like when they lie two or three moves away? For openers, consider that a working discovery requires two enemy targets: one for the unmasking piece to threaten to create a distraction (ideally the king), and one for the unmasked piece to capture after the distraction does its work. In the positions to this point both of those targets have been in place from the beginning. Now let's see some cases where one of the targets needs to be drawn onto place.
The best target for a piece that unmasks a discovered attack is the enemy king because a threat to it must be addressed by your opponent and his options will tend to be limited and safe for you. The king usually can’t escape the check by running off to protect the other piece you are threatening or by inflicting a threat of its own. It moves too slowly. So when you have some of the ingredients of a discovery in place, it’s worth some trouble to try to get the king into position to be checked by the unmasking piece. The most common ways of moving an enemy king are by checking it with another piece or by capturing a piece that the king protects.
In the example to the left, White has the queen-behind-bishop kernel of a discovery in place, with Black’s queen ready to be taken on d5 if White can find a big enough threat to make with the unmasking piece. Since White is going for Black’s queen (and since moving the bishop exposes White to the risk of QxQ), the threat the bishop makes needs to be against Black’s king to be effective. Can the bishop give check? No, not yet; White’s bishop travels on the dark squares, and the king is on a light one. So the first thing to consider is giving check with another piece in hopes of forcing the king onto a square where it can be checked by the bishop. White has one check he can give without ruining the discovery he is planning: Rh8+. Black would be required to play KxR—moving his king onto a dark square. Now White can check with BxR+; and after Black plays KxB, White has QxQ.