opening - Why does 6.Bg5 prevent 6e5 in the Sicilian Najdorf

In Gufeld's book, after 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Bg5!, [FEN ""] 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Bg5! he writes: "In this way White solves three

Sicilian Najdorf 6.Bg5

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