I'm just learning how to play the Sicilian and none of the books I have at hand shows this move. So I'm considering that this move is a mistake and there is some strong counterplay on white's part, but I just can't find it. Could anyone please point it out to me or otherwise explain the implications of 6...Nd4?
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1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 Nc6 6. Bg5 Nxd4
6...Nxd4 is not a good move because of the following reasons.
One of the reasons is tempo. As a general principle, in the openings, you do not want to make your opponent develop his pieces faster than you can. Here, Nxd4 merely speeds up White's development, since White doesn't have to "waste" a move developing his queen. His queen is already well placed on d4.
[FEN "r1bqkb1r/pp2pppp/3p1n2/6B1/3QP3/2N5/PPP2PPP/R3KB1R b KQkq - 0 7"]
Now consider the fact that Black plays 6...e6 instead of 6...Nxd4. Normally, in the Rauzer attack, White has to develop his queen on d2 to support the knight on d4 and castle queenside. Now, consider that after 7. Qd2, Black takes on d4. This position occurs -
[FEN "r1bqkb1r/pp3ppp/3ppn2/6B1/3QP3/2N5/PPP2PPP/R3KB1R b KQkq - 0 8"]
As you can see, by just waiting one extra move, Black reached the same position, but having made an extra useful move of his own - pawn to e6. So here, Black didn't loose a tempo as in the previous line.
However, despite not losing a tempo, even 7...Nxd4 might not be optimal. The reason is that the end result (of exchanging the knights) can be achieved under more favorable circumstances. This is often called "maintaining the tension" in chess. You want to maintain the tension (possibility of capturing the knight) as far as possible in order to release it at the right moment.
I will show you a popular line in which Black achieves the end result of exchanging the knights in more favorable circumstances than 7...Nxd4.
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 Nc6 6. Bg5 e6 7. Qd2 a6
8. O-O-O Bd7 9. f4 b5 10. Bxf6 gxf6 11. Kb1 Qb6 12. Nxc6 Bxc6 *
If you see the chess openings explorer statistics, in this position, White wins this position 36.8% of the time, and Black wins 35.3% as of date.
If you compare these results with the results after 7...Nxd4, the results are quite drastically different. White still wins 36.8%, but Black barely wins 5.3% of the time as of date (I have to admit not as many games have been played in this variation to get a reasonable statistic, but the fact that not many players have chosen this line as early as move 7 ought to give you a pause and make you think it's not an optimal move).
It doesn't lose by force or anything, but it brings you even further behind in development; right now White has three pieces developed to your two, and after 6...Nxd4 7.Qxd4 White has three pieces developed to your one. Also, giving up your knight on c6 weakens your control of e5, making it easier for White to push his pawn to e5 with an attack at some point. You're better off doing something constructive.
