Chess Opening Theory/1. e4/1...c5/2. Nf3/2...Nc6/3. d4/3...cxd4/4. Nxd4/4...Nf6

= Open Sicilian with Nc6 = 4...Nf6 is a move which starts the conventional Sicilian counter attack. The move attacks the e4 pawn - one of the counter attacking themes apart from the traditional Queen side attack and the break through with d5. This move also practically forces 5. Nc3 since all other moves are inferior. Now after 5. Nc3 black no longer has to face the Maroczy bind positions in which White takes control of the d5 square with c4, making the equalizing try d5 difficult or some times impossible for black to achieve.

Theory table
1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6