Black *must* calculate what happens if white grabs the bishop, so the testing line is:

A) 21.Qxe7 Nf3+ 22.Kg2 (Kh1 Qg1#) Qg1+ 23.Kh3 Qf1+ 24.Kg4 h5+ (all forcing!) 25.Kxh5 Qh3+ 26.Qh4 (Kg6 Qh7#! not Qh6+??) Nxh4 27.gxh4 Qf3+ 28.Kg5(or g6) Qf5#

A.2) 22.Kf1 Qg1+ 23.Ke2 Qe1+ wins the queen.

B) 21.Qe3 Bc5 22.Kg2 (otherwise Nxb3 wins B+R) Nc2 wins Q or Ra1 with ongoing attack.

C) 21.Qd3 (or e1) Nxb3+ 22.Be3 Bf5 23.Bxf5 (axb3 Bxe3+ wins a piece) Nxf5 leaves black a piece up.

D) 21.Q (other) Nxb3+ followed by Nxa1 leave black way ahead.

After noticing that after 20....Qxd4+ black is not gaining much, my thoughts went out to creating a powerful queen-knight battery. In this case you can immediately see it's strong, as it's a battery on the opponent's king, and the white queen is put en prise. That it costs a bishop means it should lead to larger material gain or mate.

<20....Nxd4> of course white is not forced to accept. If he does however, it will lead to mate.

A) <21.Qxe7 Ne2++ 22.Kg2 (Kh1 Qg1#) Qf2+ 23.Kh3 Ng1+ 24.Kh4 (Kg4 Qf5+ 25.Kh4 Nf3#) Qxh2+ 25.Kg5 (Kg4 Qh3+ 26.Kg5 Nf3#) Nf3+ 26.Kg4 h5# 0-1> long variation, but once you have the king going up the board with a queen and knight it is almost always mate.

Can white decline sufficiently? The battery is just as strong with the bishop still on the board. Black can try to interpose.

B) <21.Qe3> the blockage itself is fairly strong, however, black simply repeats the tactic of the puzzle <21....Bg5!>. Now white has no proper defense against the battery, and after <22.Qxg5 Ne2++ > will still lead to mate: the only difference is that 23.Kg2 Qf2+ 24.Kh3 Ng1+ 25.Kg4, now 25....Qe2+ 26.Kh4 Qxh2+ 27.Kg4 Qh3#.

thegoldenband, has answered you regarding Bxe6+ and Bxd5. But It's always nice to get an opinion from the actual player himself.

Here is Evgeny Gleizerov playing the same line (this also answers the 11...f6 prep question) way back in 1991, 14 years prior to the posted game.

Savon vs E Gleizerov, 1991

Taking the e & d pawns is just far too risky. My D.B has 20 games after taking the e & d pawns and Black has 75% wins. (it should have been higher one lad screwed up his attack and another player refused a perpetual though his opponent was 200+ points higher than him and lost - though fair do's for fighting chess.)

Lasker says grabbing either centre pawn is always worth a slight discomfort. Here you get both!

The difference being you don't keep yours as Black sacs on e5 to rip open the position and this diagram from the 1991 game.

click for larger view

Tells it's own story. Black is a piece down but every Black piece is in play and active, things looks pretty bleak for White.

Re:The puzzle:

After Nxd4 the move Qxe7...

click for larger view

...would put some lads off from going down this line OTB and they may not even stop to do a bit of calculation to see if it's on.

You have to put a lot of faith in the Dive Bomber on d4 and your ability to play the follow up correctly. (of course you also have on your side the famous rule of thumb that 3 pieces will always mate.)

You also have damned statistics.

20 ... Nxd4
21 Qxe7 Nf3++
22 Kg2 Qg1+
23 Kh3 Qxh2+
24 Kg4 h5#

If White's king instead runs toward the queen side, then we he gets to the e-file he lose the queen to ...Qe1+. The other point of this line is the f3 knight is protected by the rook.

The other try is
20 ... Nxd4
21 Qe3 Bc5
22 Kg2 (or whatever) Nc2
23 (Q moves)

White doesn't have any clever counterplay that I can see to avert or compensate for the loss of material.

20............Nxd4 to equalize.
I think this is the time that White should really think and evaluate his position

*His Queen is under attack
*His Rook is still out of play and his back rank is week

*His King is about to be subjected to a discovered check and

*His g pawn is moved and his King can be subjected to more checks

In such situation, it is not wise to leave the poor King alone and grab the Bishop on e7

Fritzie says that both win. 21...Nf3+ wins the white queen after 22. Kf1 Qg1+ 23. Ke2 Qe1+ 24. Kd3 Qxe7.

21...Ne2+ leads to a mate in five.

the game line works better! Ne2+ is superior to Nf3+ because white is deprived the option of running q side and giving up his queen to prolong the game. The following white moves are forced. It's a clean mate and nothing he can do about it:

Black's chances lie in the weak squares around the white King and especially his superior development. Therefore, giving White a vital tempo with 20...Qxd4+ 21.Be3 can't be recommended. Instead, 20...Nxd4 not only wins time by hitting the queen, but also brings the knight to an ideal attacking square.

After 21.Qxe7, Black has to avoid the greedy 21...Nf5+, because after 22.Be3, all advantage is gone and Black will be forced to swap down and fight for a draw in a simple but unpleasant ending. Key to the attack is giving a double-check instead, so White won't be able to interpose on e3: 21...Nf3+ 22.Kg2 Qg1+ 23.Kh3 Qxh2+ 24.Kg4 h5# is a pretty checkmate, and 22.Kf1 Qg1+ 23.Ke2 Qe1+ wins the queen.

Thus White's only chance is to block the diagonal with 21.Qe3. (21.Qe4 Nf3+ 22.Kf1 Nd2+ is no alternative.) Now logical follow-ups are 21...Rf3 and 21...Bg5, both with the idea to chase the queen away.

- 21...Rf3 gets a bit in the way of the knight, but it doesn't matter after 22.Qxe7 Ne2+ 23.Kg2 Qf2+ 24.Kh3 Rxg3+ and mate. White can also try 22.Qe1 or 22.Qe5 (to protect g3), but then there's no longer the need for a double check and a simple discovered check does the trick, e.g. 22.Qe1 Nc2+ or 22.Qe5 Nc6+, or 22...Nxb3+ in both cases. Black will remain at least a piece up.

Try
20......... NxP (on way to e2 forking K&B,) Kg2 (if Kh1 ==> Qg1#) 21 QxB (looks W Q has no better move)
Qc6+ (Black Q better remains in high position, going immediately to the corner does not lock the White K.) 23 Kh3 Ng1+

IF (A)
24 Kg4 Qf3+
25 Kh4 g5+
26 Kxg5 Qf5+
27 Kh4 Nf3#

IF (B)
24 Kh4 g5+
25 Kg5 Qg6+
26 Kh4 Nf3+
27 Kh3 Qh5+
28 Kg2 Qxh2#