Black pins White’s bishop to his king with the rook on g8. But the bishop is guarded twice (by White’s rook and queen—not by the f2 pawn, which is pinned) as well as attacked twice, so it’s hard to see how Black can exploit the situation. Part of the answer will be familiar from our earlier work: Black can bring another rook to bear on the pinned target by using his first rook to capture; he plays RxB+, White replies RxR, and now Black has caused White to expend one of his guards while making room to bring another attacker into play with Rg8. The pinned piece has been turned into a rook that is attacked twice and protected only once.

The only apparent hitch is that Black’s rook is pinning White’s rook along the g-file, which seems to invite White to break out of the pin with RxR. He can’t, though, because the rook on g3 is cross-pinned: if it moves up the file, Black plays QxQ. The potential for this second pin of the rook was in place from the beginning. It just was blocked by the presence of White’s rook between the pinned bishop and its queen. By playing 1. ...RxB, Black forced White into the cross-pin. White will lose a piece. (Actually there are some complications that can now arise, but they all involve losses for White and are beyond the scope of this study. White might try f5-f6+; Black can play Nxf6, inviting QxN+ from White and then recapturing KxQ. White has RxR, and the result is that Black has a queen against White’s rook.) The lesson to take away is just that a cross-pin sometimes can be created using the same logic and techniques used to create initial pins—here, by consolidating three enemy pieces into two with a capture.