The New York Stock Exchange just invoked an arcane rule last used after the Great Recession

In a sign of how badly the world has been rattled by the downturn in China's financial markets, the New York Stock Exchange invoked the rarely-used Rule 48 this morning.

Normally, before trading opens on the exchange floor, price indicators and other data are handed out to help traders determine the floor price of stocks before things get going. But if markets are sufficiently freaked out — or "volatile," to use the relevant parlance — invoking Rule 48 allows the NYSE to skip or simplify much of that process, so they can just get on with trading.

It means the designated market makers "will not have to disseminate price indications before the bell, making it easier and faster to open stocks," Dow Jones' Kristina Peterson explained back in 2010. The Dow opened over 1,000 points lower this morning, and the major U.S. stock averages all appear headed for one of their worst openings since the onset of the Great Recession.

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The current version of Rule 48 was finalized in December of 2007, and according to CNBC was last used in the aftermath of the 2008 financial crisis.