What is the winning move here and why? B1b2rk1/p3pp1p/6p1/q1Pp4/3b1B2/2n4P/P2Q1PP1/2R1K2R b K - 0 17


Stockfish rates this position as -2.94, so Black is clearly winning. Or is it so clear? After 1... Bxf2+ 2. Qxf2 Ne4+ 3. Bd2 Nxf2 4. Bxa5 Nxh1 5. Bxd5, White obtains the Bishop pair in an open position and a passed pawn. This is not the winning move. (Actually, there are two: The alternative is 1... Qxc5.)

However, I cannot fathom how Black is better. Sure, material is even in terms of points, the White LSB is locked in and eventually Ba6 will be played, but when is eventually and how?
Bxf2+ gives away all your advantage because of the precise line you mention. The correct move is e5 (to protect your bishop with tempo, note 1...e5 2.O-O exf4 3.Qxd4 doesn't work because of Ne2+ fork), followed by Ba6, preventing castling and attacking the trapped a8 bishop. Notice Bc6-Bd7-Bg4 doesn't really save the bishop as shown below: (covering some side-variations as well, rather called for as it's a very concrete position)

 [title "1...e5"]
 [fen "B1b2rk1/p3pp1p/6p1/q1Pp4/3b1B2/2n4P/P2Q1PP1/2R1K2R b KQq - 0 1"]

 1...e5 (1...Qxc5 {also a good move with similar threats except the queen discovery} 2.O-O Ba6 3.Rfe1 Rxa8 4.Be3 Bxe3 5.Rxe3 d4 {easily winning: two pieces and a pawn for the rook, together with a passed pawn and dominant knight}) (1...Ba6 {The immediate Ba6 is not good enough probably but it's a fun line, the idea with Ne2+ forking bishop and knight while still taking on a8.} 2.Qxd4 Ne2+ 3.Qd2 Qxd2+ 4.Kxd2 Rxa8 5.Be3 Nxc1 6.Rxc1 {still a pawn up but it's opposite colored-bishop so always big drawing chances.}) 2.Bh6 Ba6 3.Bc6 (3.Bxd5 Rd8 4.Bf3 Bxf2+ {wins on the spot for black}) (3.Bxf8 Bxf2+ {however white recaptures loses on the spot} 4.Kxf2 Ne4+) Rc8 4.Bd7 Rxc5 5.Bg4 Bxf2+ 6.Qxf2 Ne4+ {and you're winning the house.}