The «flash crash» was a financial snowball effect. Short selling is worthwhile if an investor is sure that a stock’s value will drop in the short term. For this reason, the price of these bonds is likely to rise as stock markets fall and vice versa. It may be necessary to hold positions for a few days or weeks to get the full benefit of any price movement, so make sure you factor in the additional costs and risks of holding positions overnight. Learn more about options trading. Still, analysts say, there is a risk of something big erupting. In the old days, a substantial portion of currency trades took place over the phone between banks.