Puzzle of the week #139

Chess Diagram: 

[Event "Puzzle #139"][Date "2010.11.01"][Result "0-1"][SetUp "1"][FEN "2r3k1/1p4pp/p1n1p3/8/P5P1/1NP3qP/1P3bQ1/R1B2K2 b - - 0 1"]

Every now and then I come across a challenging puzzle and spend a few days thinking of it. This one proved to be the toughest in a while. At the beginning I looked around and could not find anything useful to do. Black's Queen and Bishop combo seems completely disconnected from all other pieces. Moved away from the puzzle for a day or so and got back to it trying some more; still could not find anything useful. Then I asked for hints and got one:
Hint 1: Note that Qxf2 can be met by Rf8!
OK, nice observation I saw from the beginning, but how could I use it? I asked for the other hint available and received:
Hint 2: The b3-knight is vulnerable
Now this one has caught me completely off guard and led me nowhere. Long story short I could not solve it. Can you?
a) Black to move and win
b) Are the 2 hints provided useful? If you answer "No", what other ones would you choose to give?

Total available points for this puzzle is 20. The answers will be published next week together with puzzle #140.

Puzzle #138 solution:
Game: Annaberdiev, Meylis - GM Bu, Xiangzhi (CHI), round 7, 39th Chess Olympiad 2010 Khanty Mansiysk, Russia. This time we have a combo answer from Karl and Jeffrey. Nicely done guys! See solution:[/i]

[Event "Puzzle #138"][Date "2010.10.25"][Result "0-1"][SetUp "1"][FEN "2r3k1/3bpp1p/3p1npQ/4q3/rp1NP1P1/5PN1/PP5P/1K1R2R1 w - - 0 21"]{Game line} 21.Ngf5 {Karl: is a great way to start an attack. Ngf5 threatens Qg7#. Black cannot take with the g-pawn} 21...Nxe4 {Karl: is a good way to counter this attack: taking a pawn while letting the Black Queen strike g7 so White can’t checkmate} (21...gxf5 22.gxf5+ Kh8 23.Qg7#) 22.Qe3 {Karl: is retreating, providing more defenses for e4} (22.fxe4 Qxe4+ 23.Ka1 Rxa2+ 24.Kxa2 b3+ 25.Kxb3 Qd5+ 26.Kb4 Qc5+ {Black wins}) 22...gxf5 {Karl: simply taking the Knight. f5 provides further defense for e4 in case of fxe4} 23.gxf5+ {Karl: is the logical move here. It checks Black while winning that pawn back. The Black Knight on e4 can’t escape anyways} 23...Ng3 {Karl; is one way of offering a Queen trade. This sets a tiny trap: if White takes with h2, then Black goes Qxe3. There might be another reason I haven’t thought of} 24.Rxg3+ Kh8 (24...Kf8?? 25.Qh6+ Ke8 26.Rg8#) 25.Qg5 {Karl: attacks e7 and establishes a powerful battery on the g-file} 25...Ra5 {Karl: makes Bxf5 valid by adding an attacker to f5} 26.Qd2 {Jeffrey's solution} 26...Bxf5+ 27.Ka1 (27.Nxf5 Qxf5+ 28.Ka1 Rd5 {Exploiting the back rank weakness} 29.Qe1 Qc2 30.Rg1 Ra5 31.Qe4 Qb3!! 32.Qb1 Rca8 {Black wins}) 27...Rxa2+!! {This is probably the move White missed in his analysis} 28.Kxa2 Qd5+ 29.b3 Qa5+ 30.Kb2 Qa3#

Correct solutions:
Jeffrey - 18 points
Karl - 15 points
Alex - 10 points

Karl - 225 points
Jeffrey - 215 points
Rick - 174 points
Frank - 122 points
Owen - 95 points
Alex - 91 points
Edwin - 72 points
Nathaniel - 50 points