"The Two Knights Defence just loses a pawn." - Nigel Short
The Two Knights Defence is one of those openings that is seen very commonly in the amateur game, particularly in junior events, but somewhat less so in professional events. Nevertheless, I've seen games with it be crucial in such events on at least two occasions: in the final round of the 2006 EU Championship in Liverpool, Nigel Short won on the white side of it against Mark Hebden to win the tournament by half a point (the game from which the above quote is taken), and in the penultimate round of the 2009 British Championship in Torquay, David Howell beat Stuart Conquest on the black side of it, leaving himself needing only a draw in his last round to clinch the title.
So what is special about this opening? Well, as Short's quote pointed out, black must be prepared to gambit a pawn if he's playing this opening. After 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5, there are only two moves that come into serious consideration. One is the razor-sharp Wilkes-Barre variation, 4...Bc5!?, which may or may not be fully playable, but the player who keeps his head better in the tactics will probably win. This game by the current Hampshire champion is typical of the line. The rather more sober main line is 4...d5, after which play usually continues 5.exd5 Na5 (not 5...Nxd5? 6.d4 exd4 7.O-O, after which black is struggling to not get completely massacred early on) 6.Bb5+ c6 7.dxc6 bxc6, which leads to the first major fork in the road. White has to do something about the threat to his bishop, and 8.Ba4? is certainly not the answer: 8...h6 9.Nf3 e4 10.Ne5 Qd4 11.Bxc6+ Nxc6 12.Nxc6 Qd5 and white has lost a piece.
The options for white here fall into two categories. The more modern lines, 8.Bd3 (as played by Conquest) and 8.Qf3 (as played by Short), are happy to leave the white bishop on an awkward square with the intention of preventing a central advance by the black pawns. (The latter is often followed by the sequence 8...Rb8 9.Bd3 h6 10.Ne4 Nd5, and it's obvious that the idea of ...f5 and ...e4 will have a big bearing on the sides' subsequent strategies.)
The more traditional line is 8.Be2. Black does not want to allow white to calmly solidify his central position with 9.d3, so in true gambit style, he tends to open up the position here: 8...h6 9.Nf3 e4 10.Ne5 Bd6 pretty much forces white to allow an en passant capture. While 11.f4 is perfectly playable - indeed, David Spence once beat me with it -, the main line is 11.d4 exd3 12.Nxd3, after which black's compensation for the pawn is clear. He'll be able to gain a tempo with ...Qc7, and castle, develop the bishop somewhere, and put the rooks on d8 and e8.
This is exactly what happened on Monday night in the game Nash-Sandon at Barnstaple, which continued 12...O-O 13.O-O Qc7 14.h3 Bb7 (I don't like this move much, as it happens; I'd probably have put the bishop on f5, with ...Bxh3 ideas potentially in the air) 15.Bf3 Rad8 16.Nc3, reaching the position above. There's certainly plenty of play for black in this position, despite his pawn minus. His knight may be able to come back in via c4 and e5, he may be able to activate his bishop with ...Ba6 or ...Bc8 to put pressure on d3 or h3, he may even have a well-timed ...c5 and ...c4 in the air.
The clue to what actually happened in the game is the "well-timed" in that last sentence. Peter Sandon tried an immediate 16...c5?, which immediately handed the advantage to white after 17.Nb5. After that, he got outplayed in the pawn-down position. That's the risk you take with gambit play; if your timing is out, you can end up, as Nigel Short said, having just lost a pawn.
There are a number of moves that can be played after 1.e4 c5 2.Nf3; the three commonest are 2...d6, 2...e6 and 2...Nc6, but there are also a number of rarer moves that occasionally crop up. One of these is 2...a6, the O'Kelly variation.
This move is not as popular as the other three moves because black is showing his hand first; he's committing himself to a move which may not be useful against the set-ups adopted by the white player. It's no co-incidence that one of white's commonest responses here is 3.c3, attempting to show that the two interposed moves are in white's favour. Other moves along the same line of thought are the King's Indian Attack, 3.d3, and the gambit 3.b4!? with which I once beat FM Michael Franklin. Another popular option for white is to play 3.c4, aiming for an Open Sicilian with a Maroczy Bind, and this is likely to transpose into some line of the Kan.
All of these, except possibly my gambit line, are good ideas, and have their points in favour. What is not a good idea is to carry on on Open Sicilian auto-pilot and play 3.d4?!, as many players do when faced with this variation for the first time. The reason soon becomes clear: after 3...cxd4 4.Nxd4 Nf6 5.Nc3 e5 (diagram), black has got his ideal Najdorf set-up without the concessions made by having to play ...d6: his bishop can freely roam to c5 or b4, and his pawn can come straight to d5 in one go (as it would, for example, if white were to play 6.Nf5 here). (Note the contrast with the Lowenthal and Sveshnikov variations: in both of those, an early ...e5 can be met by Nb5, and white's attack on the d6 square makes it that much harder for black to get a good position out of the opening.)
Both these themes came into effect quickly in the game Hale-Owens from this season's 4NCL; from the diagram position, the game continued 6.Nb3 Bb4 7.Bd2 O-O 8.Bd3 Re8 9.O-O d5, reaching our third diagram. White could probably have equalized by swapping off a pair of pieces here with 10.Nxd5 Nxd5 11.exd5 Bxd2 12.Nxd2 (not 12.Qxd2?? e4) Qxd5; the interesting question here is whether 13.Qh5 can cause black enough hassle to make up for the slight positional advantage the pawn structure gives her.
As it happens, white played 10.Re1, to which black replied 10...d4, and from then on, it was clear that the advantage was with black. It's said of the Sicilian that black is usually doing well if ...d5 can be got in with no ill-consequences, and this certainly proved to be the case here.