Principle: There is a certain relationship between the values of two different contracts, and there is "reasonable value" or "balanced value" in the value difference.
This can come from the relationship between market supply and demand, the internal relationship of commodities, or historical experience.
When the spread deviates too much from the equilibrium value, the arbitrageur thinks it should return.
Methods: sell the varieties with overvalued prices and buy the varieties with undervalued prices.
There is also a kind of spot hedging, which is a mode for enterprises with spot or spot demand to lock interest rates/costs in advance through the futures market.