shorting futures is selling in advance. Give a realistic example. You signed a contract with someone and sold him something for 1 yuan, and now you don't have this thing on hand. At this time, you have to go to the third person to buy this thing to fulfill your contract. If the third person only sells 6 cents, then you earn 4 cents. If the third person sells 1.3 yuan, you will lose 3 cents. That means you sold something you didn't have.

the borrowing problem only exists in margin financing and securities lending.