A sentence about arbitrage in finance is called: buy low and sell high.

When the price of a security is lower than the normal price, you should buy it. Because if the market is efficient, the price of such securities will return to the normal price in the future. Since you buy at a low price, you can get excess returns when the price returns to normal.

When the stock price is set high, it should be sold or shorted, because in the case of an efficient market, the stock price will fall to the normal position in the future. If you sold it before, your loss will be less than that of the holder of this securities; If you are short-selling, it means that you need to return your short-selling stocks now. Then you can go to the market to buy back securities. The price you sell is higher than the price you buy, so you can also make money.

So if the price of stock index futures is set very low, then you should buy, not short.

Short selling means bearish on this security. If you think this stock will fall in the future, you should short it.