This is a theoretical statement, that is, when the price difference between the two exceeds a certain level, it will inevitably return to the original normal level, and arbitrage is profitable through this abnormal price difference.

The "stock index futures price" and "stock index combination" in this passage mean that the spread exceeds the theoretical value, so it will definitely return to the original normal level, so arbitrage appears. This means that you are in business. East market price 100, west market price 1 10. Then you buy in the east and sell in the west, and you earn 10 yuan. Arbitrage is the same.