Find the kernel of a horizontal discovery for White. He has his rook "behind" (i.e., masked by) his bishop on the fourth rank; he has a good target for the rook in the Black queen on h4; and he has a check to give with his bishop—two of them, in fact: Bh6+ and Be5+. Which is better? Bh6+ doesn’t work because the queen can both dissolve the check and take itself out of danger with QxB. Be5+ is better because it threatens Black’s king from a square the queen can’t reach, so the queen is lost one way or the other. If Black plays f7-f6, White replies RxQ; if Black plays Qf6, White has BxQ.