On Monday morning, when the Standard & Poor's 500 Index fell 7% and triggered the automatic circuit breaker mechanism, trading on the New York Stock Exchange was suspended. Spencer Pratt/Getty Images hide caption

On Monday morning, when the Standard & Poor's 500 Index fell 7% and triggered the automatic circuit breaker mechanism, trading on the New York Stock Exchange was suspended.

Trading was suddenly suspended on the New York Stock Exchange six minutes after trading began on Monday. At that time, the Standard & Poor's 500 Index plummeted 7%, and the market circuit breaker mechanism was activated. Trading resumes after about 15 minutes.

This was the first citywide shutdown since the stock market crash on October 27, 1997, when the Dow Jones Industrial Average fell 554 points, or 7.2%.

According to market rules, the circuit breaker is activated at three thresholds:

These automated trading suspensions are designed to prevent the market from entering a state of free fall. Monday’s suspension played a role. Once trading resumes, the main indexes will fall back from their lows. However, the index fell again later in the day, and the Dow Jones Industrial Average fell more than 2,000 points, or nearly 8%.

Stacey Cunningham, president of the New York Stock Exchange, told CNBC that the circuit breaker "is designed to slow transactions for a few minutes, allowing investors to understand market conditions, consumer information, and make decisions based on market conditions."

"This is indeed a preventive measure we have taken so that the market can slow down for a minute," she said.

The circuit breaker was adopted after the Black Monday crash on October 19, 1987, when the Dow Jones Index plummeted 508 points, or 22%.

"There is an idea that if you stop trading and pause, then people will calm down, which may stabilize the market," said Mason Gritty, an emeritus professor at Northern Arizona University and a former research economist at the US Securities and Exchange Commission.

His research in the early 1990s found a temporarily suspended trade-off. If traders expect a circuit breaker, they may sell more quickly.

"In fact, having a circuit breaker may actually make you more likely to find the circuit breaker. It's almost like a kind of gravity," Gritty said.

"People who make a living-I have never met someone who feels like they need a breather," he said. "Their lungs are good. They are used to stress."