That's because the futures market can do it in both directions. In the process of rising, you can buy positions first and then sell them for profit after the price rises. In the process of falling, sell the position first, and then buy and close the position after the price falls. Buy low and sell high, just buy first and then sell.

The target of futures trading is a standardized contract, so it can be sold first. For example, if you sign a contract with someone and sell him something in the next three months, the price will be set when you open the warehouse. After three months, the price of this thing has dropped. You can buy it at a lower price and fulfill the contract signed three months ago. That is, sell positions three months ago and buy positions three months later. Profit intermediate price difference.

The premise is that the direction of your position is consistent with the direction of price change. If you hold short positions (selling and opening positions), the price will go up, which will cause losses. Because you want to buy at a high price, it is equivalent to selling at a low price and buying at a high price.