When the price falls below the exercise price, the exercise price multiplied by leverage and then multiplied by the contract value of each point (that is, the money earned after exercise) MINUS the current price will get more points than the profit from shorting futures. You can understand that when the price falls to a certain point, the profit of buying put options is the same as that of shorting futures, so when the price falls below this point, buying put options is more cost-effective.