We have seen that a bishop can't be effective in pinning another bishop. The most common way to deal with this problem is by exchanging the pinned bishop for a better target. But there is another way as well: it may be possible to impose a second pin—a cross-pin—against the pinned piece, so that it can’t afford to capture the piece that pinned it the first time. These examples will clarify the point.
In the frame to the left, White has Black’s bishop pinned to his king; this type of position generally would arise when Black has interposed his bishop to fend off a check. Black’s bishop has protection, so playing BxB+ doesn’t get White anywhere, and meanwhile Black threatens to win a piece by playing BxB himself next move. White can't exchange the bishop for a better target, but he does have the possibility of a cross-pin. The point to see is that Black’s bishop is on the same diagonal with its queen as well as its king. White takes advantage of this with Qh7. Now the bishop is pinned twice, and both pins are doing important work: the bishop can’t take the queen because then his king would be exposed; he can’t take White’s bishop because then White would play QxQ. Nor, of course, can the bishop run someplace safe. It's attacked twice and protected just once, so it is lost next move to BxB.