With 4. Nxd4, White recaptures the pawn, and centralizes the knight. White has a small space advantage in the center and open lines for both of his bishops, but Black has the advantage of having more central pawns.

If White were to move again, he would love to play the move 5. c4, setting up the classic Maróczy Bind pawn center (pawns on c4 and e4), which also allows him to play the move Nc3 in the future without blocking the c-pawn. In addition, c4 greatly restricts Black's counterplay on the queenside, as the move makes it difficult to play ...b5 for Black.

As a result, Black nearly always plays 4...Nf6, which attacks the e4 pawn, thereby asking White the question on how he wants to defend it. The most common way to defend it is 5. Nc3, which blocks the c-pawn, thereby preventing the Maróczy Bind. Alternatively, if White insists on getting the Maróczy Bind, he may also try the move 5. f3 (the Prins Variation).

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1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4

Wikipedia has related information at Sicilian Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.