In a historic move, the New York Stock Exchange invoked the little-used Rule 48 to pre-empt panic selling at the stock market open on Monday.

The NYSE nearly never employs Rule 48 – it hasn’t been used since the financial crisis – but when the futures market is indicating an unusually ugly day ahead, DMMs need all the help they can get. That makes it “easier and faster to open stocks“, says the Wall Street Journal in an explainer “The rule was approved by the Securities and Exchange Commission on December 6, 2007”.

When the NYSE employs Rule 48, designated market makers don’t have to disseminate price indications before the opening bell when volatility is seen having a floor-wide impact.

The Dow Jones increased more than 300 points during the first few minutes when the market opens on Tuesday morning.

To invoke Rule 48, an exchange would have to determine that certain conditions exist that would cause market disruptions. The index increased more than 600 points for a short time during the premarket trading, which implied a 450-point rebound and signaled that investors ignored a deeper selloff in China’s stock market. It was almost invoked again earlier this year when snowstorms swept across the U.S.