"It's taken a big hit but it's going to all bounce back and it's going to bounce back very big at the right time," Trump said of the stock market during brief remarks to the press at the White House.

The index would have to plunge by 13 percent before the second trigger would close trading for another 15 minutes. A final trigger would close markets early for the day under what are known as market-wide circuit breakers, which regulators have put in place to curb massive selloffs when investors panic.

Pre-market trading had already shown signs of strain after Trump's nationwide address Wednesday. After the president announced a travel cutoff between the U.S. and Europe, stock futures plunged. By the time markets opened at 9:30 a.m. today, futures for all three major indices — the S&P 500, the Dow Jones Industrial Average, and the Nasdaq composite index — had dropped 5 percent, hitting the maximum allowable selloff price before the market opens.

On Monday, a break occurred within minutes of the market opening as coronavirus-related market fears collided with upheaval in the oil markets related to changes in Saudi Arabia's oil production.

Before that, the only time a market-wide break occurred was in 1997.

Today's circuit breaker rules, which went into effect in 2013, have three different thresholds to stop trading: if S&P 500 index drops by 7, 13, or 20 percent. If the market drops 7 or 13 percent before 3:25 p.m., trading will stop for 15 minutes. After 3:25 p.m., drops in those levels don’t halt market-wide trading. But a 20 percent market decline at any time of day halts trading for the rest of the day market-wide.