When the market is over-speculative, when the futures price is too high and the spot price is too low, traders short in the futures market and buy in the spot market. In this way, spot demand increases, prices rise, futures supply increases and prices fall. Finally, the spot price is delivered to the futures to earn the spot price difference.
Short selling is what we usually call short selling. Short selling refers to selling all futures held by investors (or futures borrowed from investors' accounts) and hoping to buy the futures at a lower price in the future.
When the futures market is short, brokers must intervene in futures or arrange for other parties to intervene in futures delivery.
When you expect futures prices to fall, you can buy these sold futures at a lower price. If the subsequent buying price is lower than the selling price, the net difference between them is your profit.