Rauzer Attack, Sicilian defense - why not play 6...Nxd4? 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
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.