Chess Opening Theory/1. e4/1...c5/2. Nf3/2...e6/3. d4/3...cxd4/4. Nxd4/4...Nc6/5. Nb5

With 5. Nb5 White moves to exploit Black's dark square weaknesses. When Black played 2...e6, they left a hole on the d6 square.

White has two attackers on d6 and Black only one defender, the bishop, so 5...a6? allows 6. Nd6+ Bxd6 7. Qxd6 ∓. Black gets the bishop pair and their Queen is all up in White's business.

The only way for White to add a defender to d6 is 5...d6□, allowing the black queen to see d6. White can then follow with 6. c4 or 6. Bf4.

6. Bf4 acts to provoke 6...e5, weaken the d5 square and leading the game into a Kalashnikov-style Sicilian, with Black's pawns on e5 and d6. The bishop then falls back to e3.

6. c4 is more common: White sets up the Maroczy bind structure to control d4 and taking over space.

White will have to spend some tempo retreating their knight to somewhere safer, before or after it is kicked by a6. White chooses either knights on a3 and c3, or c3 and d2.

5...Nf6 is an alternative: Black postpones White playing Nd6 by threatening their e4 pawn. After 6. Nc3 to defend it, Black can then choose to play 7...d6 after all, or 7...Bb4, pinning the knight, instead, then 7. a3 Bxc3+ 8. Nxc3 d5.

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Wikipedia has related information at Sicilian Defence

• Pavlidis, Antonios (2019). The Sicilian Taimanov. Glasgow: Quality Chess UK. ISBN 978 1 78483-058 8.
• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.