Black has the makings of a discovered attack on the f-file, where his bishop masks the capture QxQ. If only the bishop could vacate f5 and attack White’s king. At the moment it can’t be done; the bishop travels on the light squares and the king is on a dark one. Again, try handling this by checking the king with another piece, thus forcing it onto a light square where it can be checked by the bishop. Black can give check with his rook in two ways: Rd2 and RxB. After Rd2+ White can move his king to, say, g1 with nothing accomplished for Black. But in reply to RxB+ Black has to recapture KxR—moving his king onto a light square. (If White instead moves his king, Black plays RxQ—a skewer.) Now Black checks with Bd3+, unveiling a threat against White’s queen that White has no time to fend off.