With Bg5, Black can aim a bishop through two White pieces: the bishop on d2 and the rook on c1. In itself this won't do any good, so think about ways to force substitutions by making captures. Black has RxB, inviting White to play KxR. Now Bg5+ becomes wonderfully productive; the king must move off the diagonal and leave the rook behind it loose (because the g4 bishop attacks d1). After BxR, Black has netted a piece.